I've never really had this problem with Platy fry in my Jewul 110l. I guess you could put another stack of foam in to that the top grill is filled from the inside. Not sure how that would affect the flow rate tho.
You could try attaching a fine mesh using some small cable ties or thin wire, not exactly sure how easy or effective it will work.

Why not try a pair of tights! it will still allow the filter to do the job even if a little limited but of course it will only be short term until the fry are a little bigger and more wary!
I'd take your filter out drop it into some tights and then Knot it at the top aroung the power lead and push any nozzles through the tights (or cut holes) depends on what the filter mounting is like too?

