I used stainless steel mesh cut and bent into shape, it minimally impairs the flow and stops big shrimp being sucked in. The same stuff that you can buy for attaching moss:
https://www.ukaps.org/forum/threads/eheim-surface-skimmer-death-trap.31291/#post-330151
Tbh I gave up on the idea as I was cleaning it so frequently and opted to just pick out the silly shrimp when the sponge got clogged.
There isn't really a shrimp-safe surface skimmer design, maybe an overflow box/down-tube
Any shrimp/fry protection measures will always compromise surface skimming ability though. Up to you if the trade off is worth it, it definitely is if your baby shrimp are valuable to you and you don't mind more frequent cleaning.