I suspect the issue with keeping the trailer at 8'6" max width is that the reinforcement needed for the sides/ceiling would impinge upon the interior width, so your 8'6" cat wouldn't fit.

Unless you "jake-i-fied" the trailer so your cat slides out from the side where there would be no supports needed.. They'd be on the four corners or just ahead/aft of the hulls and crossways along the ceiling.

So maybe an 8'6" width by 22 ft. to accommodate the frame members and a roll-up door on the side? And maybe rails (or an overhead gantry type thing) to slide the hulls out laterally (where the beach wheels would be sitting).

Oh yeah, it all works out great in my imagination....