Glad you asked. I used some 500 micron mesh to make a grill. 500 micron = 0.5mm * 0.5mm holes. I normally go 1mm mesh for CRS but since chameleon shrimplets are supposed to be small, I thought it better to go to 500 micron.
I made a frame using pieces of zip ties and glued it onto the mesh using super glue.
This side sits tight against the back of inlet (facing into the tank) so that the mesh is flush with the inlet holes.
This side (the frame) holds the mesh flat against the inlet holes. As you can see, I made the mesh wider than what would fit then cut it down to size using a hot knife. It fits really tightly, so it wont be going anywhere and I wont have to worry about it falling out of place.
Here is how it sits into the filter. I push it down so it sits flush with the top of the filter. Hey presto.