Briananemone - Go into a plastic fabricator. there will be dozens around, people who work with plastic to make signs, stationary holders etc. Go in with your measurements, and they will cut it to size, be surprised if it costs $10.00 if the can find it in an off-cut.
One of the places down here had shelfs and shelfs of offcuts, I think I did them a favor by taking some away
My overflow cost $15, the plastic (blue), cut to size, cut in half, bent and 90 degrees, with 3 extra bent strips and they even threw in some glue in case I needed to glue some together. Awesome. Laser cutting cost an extra $60 I think, but had to be done at a different place. worth it I feel, as it just looks tidy compared to DIY (which I have on my other tank
Pie