Why do you want a net cover on your fish tank?
Most people have coverglass made from sheets of glass or Perspex to stop fish jumping out and to prevent evaporation. Coverglass also stop dust getting in the water and prevents water splashing onto lights.
If your tank is 18 inches wide, get some coverglass made that are 18 inches x 12 inches wide x 4-6mm thick. Thicker glass is more expensive but less likely to chip compared to thinner glass.
Have 4 individual sheets of glass for the coverglass. You can cut corners out for airline or filter pipes. You can cut a larger triangle section off the front corner of one piece and put a couple of supports and a small handle on it so you can lift it up to feed the fish.
If you have concerns about glass, use Perspex, which is a clear plastic sheet. Get Perspex that is 4-6mm thick and glue some ribs to it so it doesn't sag in the middle.