Plexiglass Lid

[Kabong]

I use plexi on my 50. So 2 1/8 pieces about 18" by 18".
It has a center brace so it only spans about 12" unsupported.
It does bow a bit but not to the point where it touches the water or anything.

If you got a dremel or a rotozip and a drill its simple to work with and you can custom cut it to exactly what you want. As well as cut out spots for filter's, light legs, cords.