I was having this same problem with hermits and snails, not to mention the odd fish taking a leap once in a while
So last weekend I finally did something about it
I went to Home depot and picked up a sheet of egg crate and some fiberglass window screen. I simply cut the egg crate to fit my overflow, then wrapped it in the window screening and used some frag glue to hold it to the egg crate. I then placed three suction cups just under the overflow slits, so the the new screen sits just below the water fall and is held up by the suction cups. If anything now ventures to the "dark side" it can simply flip, crawl, jump, or wait for a helping hand to retun back to the tank. I also have a large grade of pond filter in front of the bulkhead, that way in the past or present, no fish, critters, or now suction cups will be able to clog up the bulkhead hole or pass into the sump.
I will try to take a picture of it tomorrow to give an idea what it looks like.
It works great already, as I saw a snail crawling on the screen earlier and then it made it's way back into the tank. The screen has also not impeeded the flow either. It took me six years to finally make this minor adjustment to the tank, like I partioned my sump last year too. Sometimes the little improvements that we do to our tanks can make a big difference
