Anyone have any idea how to modify an ARS cb70 slot to keep Escape artist Rubber Boas secure?

I'd like to Mod the slot with out altering the ARS 7010 Rack.

I was thinking some type of 1/4" flexible plastic molding around the tub as a shim to reduce the gap to just enough to slide open the tub?

A plastic shim of some type affixed to the rails on either side to reduce the gap to just enough to slide the tub in and out?

I may have to put a sheeet of plexi-glass over the top to prevent small ones from exiting through the vent holes? I guess I could unibit smaller holes in the Tub itself?

Keeping them in a Slot in a Rack would make maintenance easier at this point . Thanks for any ideas. I'm stumped.