As Pinkskunk mentioned, BTAs can fit through the smallest openings. Eggcrate has limited use.
Depending on how much space you want to dedicate to the BTAs, and if you're okay with the how it looks, you can also just float a large plastic strainer in the frag tank. A few LFS in my area keep their nems this way.
If aesthetics are a concern, you're probably better off getting a piece of acrylic and drilling small holes in it (think of a drip tray on a wet/dry) and using it to section off the nems. I tried using eggcrate and it only works for big nems (I have gigantea). I also tried adding window screen material over eggcrate, held in place with zip ties, but I found that it collects detritus and waste on the screen itself -- I think the holes are a little too small.