Bill, I used a cabinet door roller latch to fix mine. The latch cost $2 and a couple of 4-40 screws with locknuts.
I installed the catch so the top of the roller is 15mm from the top edge of the cover. Snaps in and out easily and holds nice and tight.
BTW, the cover makes a great place to mount your E-Z PASS. Out of sight and works great.