does your cabin still have the llama skin intact? not really sure if there is a layer of glass on the bottom, just resin soaked wood. you also have the framing for the sliding hatch. i think jj in PR had the same issue at one time and he just added additinal bracing underneath.

That might work well??

But you will have to make sure you get the the ply to bend to the shape of the deck, and filled the right way so there is no pockets under the finished top.
From doing from under you avoid all them issues as any imperfections will be under and never seen.

You could also do from below and use some sheet rock screws from top to screw ply up, Then when finished remove screws from top and just need to fill the small screw holes

Now there's a possible drawback. If the bottom is just resin soaked wood then my approach will defnitely not work.

Let me check into it and report soon enough.