You can get plastic light diffusion panels at Home Deport. They are
2'x4' panels with a 1/2" grid that fit into a drop ceiling to use with
flourescent light fictures. The pieces break very easy with a flat
file. Cut them to a size so that you leave about a inch clearance in
the tank. Cut 1" (two squares) wide strips to go around the edge. File
them smooth. Using PVC cement (for glueing PVC pipe) glue the strips to
the panel so the panel sits on top of the strips. Depending on the size
of the panels you may need to add a few strips in the interior of the
panels for added support. Generally one strip every foot. Once this is
finished you can glue fiberglass window screen to cover the plates.
Trim the excess screen. This works well with drilled tank bottoms as it
will fit over the bulkhead fittings. You will need to make holes in the
panel to fit return tubes or lift tubes. Just break out enough square
for the tubes to fit through. You might want to clue a square of
plastic with a hole drilled to fit the tube over this hole in the grate
to provide a better fit so that the gravel does not get down through
the hole under the gravel.

I've made scores of these for custom tanks. They are easy to make, but
you may have trouble at first. They are very messy to make as glue is
dripping everywhere when you glue the screen to the plates. Use lots of
glue but do it on a surface that you don't care about.

If these are for a standard tank whose bottom is not drill you will
probably be better off just buying a stand undergravel filter for it
though. I would not use these, or any other undergravel filter on a
planted tank though.