My girlfriend's aunt has had that exact
same problem (she's also got the 29 g cube with FW
, it was given to her as a gift). I tried to fix it for her by buying some filter floss and just ripping it up and kinda molding it onto the edge between the tank and the filter part (so it sits right on top of the ridge). That was only two days ago, so I don't really know how well it works, but as far as I know, no more fish have gone over, and it looks like it would at the very least be more difficult for them to make the jump...
Also, somehow one of her glofish made its way into the filter assembly on the bottom right side of the thing and it's in an area that can't be disassembled so the best she can hope for is it can somehow make it out of there on its own (she thinks since it's so small it went through the slits)
You can give the filter floss a try, or hopefully someone on here can give you a better solution..