How big is the tank, and what do you plan to keep in it. Any lighting/or canopy planned? I ask because leaving your top open [egg-crate] will speed evaporation, and if you will have a large fish it will splash water [even if only at feeding time.]
Thin Plexiglass will sag, and the thicker stuff is expensive.
I once made a tank top for a friend using a single piece of 5/8" plywood.
I cut it to fit the top of the tank, jig-sawed out 3 openings, sanded smooth, hit it with 2 coats of Krylon. Then I used 1/2" self-cutting stainless-steel pan-head screws & 100% silicone to fix/seal the cheap [...like 1/8"] plexi to the bottom. The plywood frame was nice & heavy to keep his fish from knocking it off, and kept the cheap acrylic from sagging too much.
For lighting we installed a $15 track-lighting 4' track on the ceiling, and hung 3 cheap [like $9.99 each] but nice looking blue glass pendant-lights, 1 over each opening at about 6" above. We used 3 x 60W CFL [really 16W each] bulbs, and they provided good light. The whole project was under $100, but looks awesome and matches his apartment well.
What I would change if I did it again, is that he has to lift the entire lid and slide it back and up, propping it against the wall behind the tank when cleaning the tank. I had a 'Duh...' moment obviously. Offsetting the openings towards the rear, and hinging the front 4-6 inches would have been a good idea...