Reddit reviews MCG Textiles Needlepoint Interlock Canvas 18"X20" -14 Mesh White

possibly just some of that plastic needlepoint canvas

or the plastic mesh net stuff from bath poufs
If you cut the stitching holding them together you get a million feet of stretchy mesh netting that should let your eggs fall through but not your tetras. I use the stuff for netting down moss to stones.

They make HOB breeding boxes that have this feature built in, however.

like this one

or this one