I have been thinking about you question for a bit and have two suggestions. 1) If the inflatable is on top of the cabin all the time why can't you just ratchet strap the kayak on top of it. It would make a nice cushion for the kayak. 2) Build a rack like you see on Pickup trucks to haul lumber on the top of your cabin, the inflatable will sit under and the kayak can be strapped on top...
Hope this helps, sometimes you just need new eyes to see things differently.
A couple of funoodles the ones with holes, some straps and you can tie it on where you like.