If there's interest in the long rambling answer, let me know and I'll come back with additional thoughts.
Yep just when you get time, I'd be interested in hearing what what you have in mind, no hurry though just when you get a chance is fine.
Options I've considered:
#1: DIY with acrylic sheets - The problem is that acrylic absorbs water, so thin sheets will curl after time. Thicket sheets will resist this, but thicker acrylic costs more. I've heard of using something else in addition to the acrylic to act as a skeleton and help keep it straight, most often "egg crate".
#2: DIY glass tops (or buy some from Over The Edge). Depending on the bracing a tank has, you may either have to get some really large tops or add some bracing to support multiple sections of tops across a non-center-braced tank. The only other downside would be the weight IMO. It is possible to buy the hinges, glass and plastic backing (if you needed cutouts).
#2.5: Less fancy glass tops - just use flat sheets of glass. Main downside is it's harder to lift them up for feeding, but you can either purchase slip-on plastic handles or silicone a marble/other object to make them easier to lift.
#3: ABS plastic sheets - these would remain ridged and keep fish in, but are opaque and would block light. Perhaps just using them for portions of the tank that don't have a light immediately above might work.
#4: Styrofoam - easy to work with, may be messy depending on the type of foam, but also opaque.
#5: "Egg crate" - would keep fish in, let light pass, but would allow max evaporation (thus I only ever use it as a temporary cover until finding something better).
#6 Solid, transparent lighting diffuser - There are a bunch of varieties, allowing light to pass in different ways. I'm not sure how easy it is to work with. I'm not sure if it'd warp, but I'd suspect not. It might mess up the shimmering effect of LED lighting depending on the style.
Right now I'm tempted to go with either glass tops or the solid lighting diffuser.