I was thinking I'd make the boards and the bracing for it, then cut the polystyrene sheet to fit to the underside of the plywood board, between the framework, to try and stop the amplifying effect. Any opinions on if this would work? Or could cans of expanding foam be used to fill the voids and quieten it down?
And the 2nd part of this, a simplistic way of making the baseboards. The shed I bought is actually a potting shed, so it came with a sturdy bench that goes all along the front. Construction is simple, rectangular frames that are screwed to the floor, and to the wall, and then long lengths that run along the length of the shed. I was planning on mounting the baseboard to the top of this, and copying the construction method on the other side of the shed. Photos attached below to try and show how it is put together (I've not actually screwed all the supports in yet, hence the 1 that's turned round the wrong way).

