That's not a stupid idea, but my only concern would be them losing buoyancy over time due to whatever reason. The drip tray idea will actually be beneficial to this project, so might as well go that route.
Some pictures of how I'm cutting the plexiglas, if anyone had wondered.
40-tooth blade on the table saw, pushing through with the mitre gauge.
Sacrificial DIY-clamp for pieces that get my fingers a little too close to the blade.
Nice clean cuts all around by keeping the blade low (height = 2x thickness of material).
I also ordered the glass yesterday, ready on Wednesday! Ended up going with the full-length 'coast-to-coast' overflow box - I can drill through if necessary for the returns.
What I'm currently pondering is what is plastic but also threaded, and cheaply/easily available...? I had thought maybe something like those clear curtain rods, but they might not deal with torsional forces involved with threading. The other end of this would be stainless steel, but that's probably more cost than I'm prepared to pay.