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.
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Tim
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