Seems like there's always a few who can figure a way out, not to mention getting knocked-over. A lid off a large coffee can is just a tad small, they'll go if stretched, but can pop-off on their own. I know they make different styles, including the tube type with the funnel end, but I've always liked this type better (they stay put in the boat).

I cut a piece of 3/4" foam to fit inside the sleeve, then a piece of 1/2" foam the same O.D. as the cage, glued them together, and added a piece of Velcro for a latch, and a zip-tie for a hinge, then glued a coffee can lid on top. Now the jumpers/climbers can't escape, even if knocked-over.