Not window screen.
Bird netting used for ponds.
It's as thin as the lightest of fishing lines. If you place the netting to alternate with the intersects of the eggcrate, it becomes almost foolproof, even for smaller fish. Trust me, you'd be amazed at the size of a fish that can squirt thru eggcrate and end up sitting there for you to peel it off. Tacking the netting to the bottom of the eggcrate is MUCH easier than building the window screen frames to support it by itself.
I'll say it again:
Eggcrate does not impede lighting. It focuses light straight down. Take a look at the way the stuff is made it has a thicker side. The stuff is made to sit in a ceiling grid light fixture, so that it can grab light from the tubes and focus it downward from the body of the fixture.
In an aquarium setting it does seriously constrain the area your lighting covers. So a two-tube fixture just doesn't cover the same amount of real estate in your tank.
This subject was beat up mercilessly in the saltwater side of the hobby years ago. The example of the windowscreen frame above is just one of many, many.
The choice of eggcrate, eggcrate with netting, or window screen with netting is simply a function of what you have in the tank. Jumpers, lighting requirements, etc. etc. The window screen frame is the most time consuming solution, so if you change things down the line, you may have the thought of how long it took to make in the back of your mind. Just a bunch of variables to consider.