It'd be nice if something like that would work- I would rather not silicone it directly to my tank (my filter will be built into it)
Ok, I'm back.

I had come across a few threads where they talk about siliconing sand to the bottom of the tank to get the sand look without the sand hassle, and this idea occurred to me - and I hope I can clearly explain it.
Background:
Base your 3D background on a 'back plate' of ~1/8" acrylic that snugly fits between the tank bottom and the tank rim. Add whatever you want to it, but leave the bottom 1/2" of 'back plate' exposed.
To help 'lighten it up' (which actually just makes it less buoyant

), I think I would follow the video tutorial's approach, but I would match the front and back lines so that it is 1" thick (or probably 1/2" or so) throughout, which would then be attached to the 'back plate.'
'Floor Box:'
Build a 'floor box' from aquarium-safe plastic, maybe 1/2" tall, with four sides - top, bottom, front & back - with open 'ends' - that fits snugly in the tank except for in the 'width' direction (not length, but front to back); leave it 1/8" narrower than the width of the tank, so that there's a 1/8" gap at the back of the tank.
You could add texture to the 'floor box' by layering additional pieces of acrylic or whatever, you could silicone sand to the floor box, or add 'lips' to hold sand to be added, or even leave it bare.
Assembly:
Install the 'back plate' and background on the back of the tank. Put some rocks or other safe-but-heavy items (tiles, etc) in the 'floor box', and then slip it in to the tank, and fit it against the 'back plate' and under the 3D portion.
The 'floor box' will keep the bottom of the background against the back of the tank.
The only thing left to figure out is how to ensure that the top stays under the rim - there are a few options here, and I'm not sure what would work best. If we could find something that's very secure but 'disconnectable,' then you could quite easily make the floor box a bit less 'snug' and then just disconnect & slip the background out.
Using my 29.5"-long tank with 9" of water depth and an average foam thickness of 3/4", there would be about 0.00326 cubic meters submerged, which would generate a
buoyant force of about 7.2 lbs - I can't see that being too much for the rim to handle.
...I'm pretty sure that this setup should actually work - at least in theory.
