I've been keeping marine aquariums for over 30 years now. I've used pretty much everything you can imagine as a cover at one point or another over the years.
1. Glass Tops. My first 55g setup came with them so thats how I set it up. It was a great place to sit my DIY T12VHO lights, and the plastic section in the back was easy to cut for power cords and a HOB filter. It worked fine but was a pia to keep clean.
2. Acrylic. My 60 cube came with glass tops as well and they worked until I dropped them. Then? Not so much! I replaced it with 1/4" acrylic that I braced on the underside with acrylic "joists" to help prevent warping. It still did a little, but it was good enough. Again, easily modified and a great place to sit my DIY lighting on.
3. Eggcrate. At the same time I had the 60, I had both a 120 and a 150 long. Neither came with tops and I was tired of messing with glass/acrylic so eggcrate it was. The 120 was a mixed reef so I inadvertently help increase oxyenation. I'm sure the corals were happier for it. The 150 was home to a 4' Moray who liked to escape. The eggcrate was held in place with the DIY lights and chunks of dry rock in all corners. WIthout that he was on the floor.
The downsides to eggcrate, IMHO, are that it is actually a lighting diffuser, not something we necessarily want for our reef tanks, and at least IME, gets brittle with age.
4. DIY screen tops. That leads me to my current systems, a 120DT and a 60g FT. Both have DIY screen tops made using HomeDepot window screen frames and 1/4" mesh from BRS. To allow for cordage, I made the DT tops in two sections. The back section is just wide enough to use the corner brackets, and small slits in the mesh allow for passage of cords, then sort of self-heal around them. The front, larger pieces are easily removable for maintenance. Both can be taken to the utility sink and sprayed clean using nothing more than hot water.
HTH!