I wouldn't recommend plexiglass/acrylic because it does WARP. Also one thing I'm fully aware of NOW, is it's not strong enough to use as a lid especially for a HEX style tank.
The types of plexi that are STRONG enough, are 4x more expensive, so glass is much cheaper if you can CUT it.
I think I have an idea of the size HEX you have, but glass is the BEST solution.
I was going to recommend the glass shop, but I thought you would want a DIY solution.
You would need 2 pieces of glass. 1 for the front & 1 for the back.
Unless you want to remove 1 WHOLE piece of hex cut glass to feed your fish.
Cyroplast is not good because it's not strong enough & doesn't allow light to penetrate.
Also if you're headed to Home Depot,
- CarbonX mentioned you can get a 2 pack of Philips Brand = T12 - 24" T12 fluorescent 6500k lights for $10.
- I found a decent source for 6500k CFL's 100W equivalent bulbs for $10 for a 4 pack.
Personally..... I'm going with the 100W CFL's because I prefer BRIGHT!