I have one of Smiz's custom made tops for one of my nano tanks and it is very nice. I like that he can put a separate opening for feeding and dropping in your cleaning magnet. My light fixture has side mount brackets and sits on the side of the tank, so conventional screens don't work too well.
I will say one thing on the eggcrate suggestion. If you have small fish like wrasses and blennys, the 1/2" openings are large enough to allow them to still jump. I have lost several fish trying to use it. One Lawnmower blenny was dried up on the floor the first morning after I installed the top. It is not a reliable cover for fish that will fit through the 1/2" openings.
I will share what I did on the covers that I currently have on my 140. I have over the top returns at each corner for my closed loop system and two more returns from the sump, so a regular rectangular cover would not work very well. What I ended up doing was to make two rectangular tops to fit inside of the area that it would fit. That left a strip of a couple of inches at the front and back of the cover. So, I attached eggcrate to the front and rear edges of the tops, to fit around the returns. Since the holes in the eggcrate are too large, I then attached gutter guard to the underside of the eggcrate. These tops have worked out really well.
If you has a bow front tank, you could make a cover similar to the pics below, but attach gutter guard or the 1/4" mesh to the bottom side of the eggcrate.
The gutter guard attached to the underside of the eggcrate panels. This really doesn't block light, since my LEDs are mounted down the center of
of my canopy. These strips are also at the very front and rear of the tank, so nothing really underneath these panels anyway. The rest of the covers
are made with the screen kit rails and the 1/4" mesh from BRS. Just another idea to consider.
My Smiz top on my DIY nano...