I saw a video some months ago with a guy putting out ideas on how to increase the pack size from 80%. I have 3 Spirit packs and was toying with ideas to mod their size.
The Rubies back is foam from what he said which would be easier to cut.
I have a couple Dremels with cutting discs that should do the job on the Styrene Spirit packs.
He suggested cutting the pack in half horizontally at the top of the cyclotron .
So once that is done I would think that it would need a couple pieced or squared off plastic or wooden rods inside the shell at both side edges for strength and patch the gap with some styrene sheet plastic.
So that puts the pack at the correct length. So next would be to figure out how to widen the pack.
The cyclotron would be another issue. Anyone have any ideas?