Yes I used these instead of tennis balls: Diversitech MP4-E E.V.A.... https://www.amazon.com/dp/B00BVEMLR4?ref=ppx_pop_mob_ap_share

Plywood would totally work, or any slab of wood that is at least an inch thick; still, salt finds a way so that would be my only concern. You could try treating it with a nice thick coat of waterproof lacquer. That would significantly cut the overall cost. As far as anti-vibration pads, these ones are legit and are the ones we use in our float center: https://www.amazon.com/dp/B00BVEMLR4/ref=cm_sw_r_cp_api_nrEGBb7D787DX

They come in packs of 4, so you would need to order 3 of those (for a total of 12). If you’re worried about the wood splitting you could order 4 packs and be completely set.

If you end up making the anti-vibration pad I’d be stoked to see the finished project, if you don’t mind following up with another post.

Good luck with your float adventure, and congrats on the new tank!