I made a copy of the MFT-A, removed the VolumeContainer but kept the rest. Now I can only choose between the kerolox and hydrolox body variants, not the the cryo or the cradle thingy version. Is this linked to having SSTUVolumeContainer?

No, it is caused by those two body variants being added through an MM patch that explicitly targets the MFT-A part. If you duplicate the MFT-C/CF patch files, and retarget them to your new part, it should all work.

Edit: Link to one of the patch files: https://github.com/shadowmage45/SSTULabs/blob/dev/GameData/SSTU/Parts/ShipCore/Tanks/SC-MFT-C-TankPatch.cfg

AAAah... okay, thanks!!!!

edit: just added them... Just a question, is there a reason it's not integrated into the MFT-A file? (ignorance is bliss).

Its not integrated as I was sick of writing 1/2-mile long config files (and scrolling through pages of configs), and found it easier to read if I kept it in a separate file as a patch.

Could it be integrated -- yes. Should it be integrated -- sure, probably wouldn't hurt now.

:) I thought as much :)

If you wanted to throw together a PR that merged that stuff in, I would likely accept/merge it :)

Will do! ;)