i made mobile platforms almost a year ago, perhaps for the same reasons you want to. You can take a scout frigate and change the hudicon, the smallHudIcon and the infoCardIcon. Use the weapon the defense platform uses, such as beam defense. Change the .mesh to point to the planetmodule mesh and then add a custom string, like mobilebeamdefense in the string folder, for the name and description of your new mobile defense to differentiate it from the original.
There is no way to disable the phase jump that i am aware of. That is determined by the frigateRoleType, which in this case if we are using a scout, then it can still jump. I did in my first tries use the moduleConstructor instead of the scout, to keep it in the well, but since i had "defined" it as a moduleConstructor, i had to leave in the "build" abilities, which made my defense platforms also constructors. That was cool for a while, but then later i wanted them to jump, so i used the scout framework and was able to get them to jump.
the code i used for the Advent MobileDefense is here:
TXT
entityType "Frigate"
defaultAutoAttackRange "GravityWell"
defaultAutoAttackOn TRUE
prefersToFocusFire TRUE
autoJoinFleetDefault TRUE
canBomb FALSE
frigateRoleType "Scout"
statCountType "FrigateScout"
mainViewIcon "MAINVIEWICON_PLANETMODULE_PSIORBITALBEAMDEFENSE"
picture "PLANETMODULEPICTURE_PSIORBITALBEAMDEFENSE"
NameStringID "IDS_PLANETMODULE_PSIORBITALBEAMDEFENSE2_NAME"
DescriptionStringID "IDS_PLANETMODULE_PSIORBITALBEAMDEFENSE2_DESCRIPTION"
counterDescriptionStringID "IDS_COUNTERDESC_FRIGATE_NONCOMBAT"
basePrice
credits 250.000000
metal 0.000000
crystal 0.000000
slotCount 2.000000
BuildTime 35.000000
hasLevels FALSE
ExperiencePointsForDestroying 20.000000
MaxHullPoints 1700.000000
MaxShieldPoints 800.000000
HullPointRestoreRate 2.000000
ShieldPointRestoreRate 1.000000
BaseArmorPoints 8.000000
maxMitigation 0.600000
Prerequisites
NumResearchPrerequisites 0
numRandomDebrisLarge 0
numRandomDebrisSmall 3
numSpecificDebris 0
armorType "VeryHeavy"
hudIcon "HUDICON_PLANETMODULE_PSIORBITALBEAMDEFENSE"
smallHudIcon "HUDICONSMALL_PLANETMODULE_PSIORBITALBEAMDEFENSE"
infoCardIcon "INFOCARDICON_PLANETMODULE_PSIORBITALBEAMDEFENSE"
minZoomDistanceMult 2.500000
NumWeapons 1
Weapon
WeaponType "Beam"
damageEnums
AttackType "ANTIMEDIUM"
DamageAffectType "AFFECTS_SHIELDS_AND_HULL"
DamageApplyType "OVERTIME"
DamageType "ENERGY"
WeaponClassType "BEAM"
DamagePerBank:FRONT 180.000000
DamagePerBank:BACK 0.000000
DamagePerBank:LEFT 0.000000
DamagePerBank:RIGHT 0.000000
Range 7500.000000
PreBuffCooldownTime 9.000000
CanFireAtFighter FALSE
SynchronizedTargeting TRUE
PointStaggerDelay 0.750000
TravelSpeed 0.000000
Duration 4.000000
WeaponEffects
weaponType "Beam"
burstCount 1
burstDelay 0.000000
muzzleEffectName "Weapon_PsiCapitalBeamHeavy_Muzzle"
muzzleSoundMinRespawnTime 0.100000
muzzleSounds
soundCount 1
sound ""
hitEffectName "Weapon_PsiCapitalBeamHeavy_Hit"
hitHullEffectSounds
soundCount 1
sound ""
hitShieldsEffectSounds
soundCount 1
sound ""
beamEffectSounds
soundCount 1
sound "WEAPON_PSICAPITALBEAMHEAVY_TRAVEL"
beamGlowTextureName "PsiCapitalBeam_Glow"
beamCoreTextureName "PsiCapitalBeam_Core"
beamWidth 30.000000
beamGlowColor ffffffff
beamCoreColor ffffffff
beamTilingRate 3.000000
m_weaponIndexForRange 0
hasWeaponLevels FALSE
mass 3000.000000
ShieldMeshName ""
maxAccelerationLinear 125.000000
maxAccelerationStrafe 9.000000
maxDecelerationLinear 500.000000
maxAccelerationAngular 2.000000
maxDecelerationAngular 9.999996
maxSpeedLinear 700.000000
squadTypeEntityDef:0 ""
squadAntiMatterCost:0 0.000000
squadTypeEntityDef:1 ""
squadAntiMatterCost:1 0.000000
maxNumCommandPoints 0
NumSoundsFor:ONATTACKORDERISSUED 3
SoundID "FRIGATE_PSIAA_ONATTACKORDERISSUED_0"
SoundID "FRIGATE_PSIAA_ONATTACKORDERISSUED_1"
SoundID "FRIGATE_PSIAA_ONATTACKORDERISSUED_2"
NumSoundsFor:ONCREATION 1
SoundID "FRIGATE_PSIAA_ONCREATION"
NumSoundsFor:ONGENERALORDERISSUED 3
SoundID "FRIGATE_PSIAA_ONGENERALORDERISSUED_0"
SoundID "FRIGATE_PSIAA_ONGENERALORDERISSUED_1"
SoundID "FRIGATE_PSIAA_ONGENERALORDERISSUED_2"
NumSoundsFor:ONSELECTED 3
SoundID "FRIGATE_PSIAA_ONSELECTED_0"
SoundID "FRIGATE_PSIAA_ONSELECTED_1"
SoundID "FRIGATE_PSIAA_ONSELECTED_2"
NumSoundsFor:ONSTARTPHASEJUMP 1
SoundID "FRIGATE_PSIAA_ONSTARTPHASEJUMP_0"
MeshName "PlanetModule_PsiOrbitalBeamDefense"
ExhaustParticleSystemName ""
ExplosionName "Frigate1"
HyperspaceChargingSoundID "HYPERSPACE_CHARGEUP"
HyperspaceTravelSoundID "HYPERSPACE_TRAVEL"
EngineSoundID "ENGINE_PSICAPITALSHIP"
ability:0 "AbilityAdaptiveShield"
ability:1 "AbilityBoostMorale"
ability:2 ""
ability:3 ""
MaxAntiMatter 800.000000
AntiMatterRestoreRate 1.500000
cargoType "Invalid"
maxCargoCapacity 0.000000
formationRank 2
minShadow 0.200000
maxShadow 1.000000
Check the code against the original planetmodule and you will see the differnce, or against the Advent Scout. I used the same principle for the Tec and for the Vasari using the Gauss defense and the missile defense for the Vasari.
If you are doing this for Entrenchment, of course there is the entity.manifest that you will have to consider. Basically you will add a new frigate (alphabetically in the psi list) and add to the number at the top to account for the additional .entity file you added.
Of course this means if you make one for each race, you will add 3, with a frigatePhaseMissileDefense and a frigateTecGuassDefense and frigatePsiBeamDefense in the body listed alphabetically. Some others have been able to add things it appears non-alphabetically, but i get an error when i dont, so i do it that way.
If you want to look at them the way they are finished you can check out my NightFall v1 mod for Entrenchment, or my OverLords v1,6 mod for Vanilla Sins.
NightFall v1
OverLords v1.6
There were other changes i had gotten to by the time i did these, but you can simply take the code for each and use either Entrenchement or Vanilla to copy them into leaving the rest of your game the same.
As a side note, you may want to download Dekhranic's converter to convert the binary files over to text to edit them, and Newbeorg's resize tool, which if you get any ideas about re-sizing larger or smaller the standard meshes, then that will be a HUGE help. 
Hope all goes well, take care and keep modding!!!
If there are any questions of course feel free to ask, i'm not online as much as i used to be, school is heavy this semester, but i'm on every few days easily.
Let me know how it turns out for you.
-Teal
