Didn't fine any eligent solutions to this problem that retained the ball check valve used for priming the filter. So I made one.
1.5mm holes
Print with ABS or PETG. I tested both. Settled on ABS as it looked nicer. Pictured is the PETG version. Both seem to be quite strong. Was not able to break it by crushing it with thumb and index finger.
May need to scale the print in the slicer to 99-101% to get a snug fit depending on your printer.
Tested for a month. Found no shrimp in filter with over 200 in the tank. They are almost always on the strainer eating off it. I would keep the intake away from anything that might have baby shrimp hiding in it. Strainer did not restrict flow or clog easily. Definitively don't need to clean it as often as a sponge pre-filter. While testing filter was at 100% flow.
According to the Fluval website this will fit the following filters.
305/405 Canister Filter
306/406 Canister Filter
307/407 Canister Filter
Someone with a little bit of CAD experience should be able to modify this to fit the other models by making either an adapter ring or making the attachment point smaller. Scaling the part might work but it could make the holes too small.
The author marked this model as their own original creation.