Ship design Question

How do you stop something from rotating?

The piece in question is the Supply Module from the Raider Fleet set for a custom race. The one available for the design set counter-rotates two double-ended cargo boxes around a central shaft. When not spinning they are a perfect fit for a GP Cargo Hull I designed.

I can get them to stop in the design screen when I attach them and then save. But when I click on it on the main design window it pops up and the screen and the parts are rotating through, well, the rest of the ship. A major design problem. How do I stop the spinning? I don't need spin; I have artificial gravity.

Edit:

I incorrectly said this was the Cargo module; it was the supply module and I can't use it where I wanted anyway. Please disregard this question.