I don't know if the lip on these are wide enough for you, but these are the type net pots I always use. I just make sure I don't cut the holes I put them in to big, and they have always worked fine for me. I generally just flip it over and draw the outline of the top lip, then cut it out about 1/8 of an inch smaller than the line. For cutting it out on rounded surfaces, I create a template the same way I mentioned, but on a thin piece of plastic (margarine or whip cream container lid etc.). Then it's easy to draw the line on any surface.
3" Net Cup
Net Pots
If you have a hydro shop near buy, you may be able to get them in quantity there and not need to pay shipping. A few years back when I was in Las Vegas I got a 100 pack of the 3 inch net pots for about $25.