hikermor.. would either of the following work?
1. cut circular lid of aluminum sheet slightly larger than diameter of lip... 3 tabs located at 90/180/270 degrees from each other, with a "J" lip(opening outward) formed... slide lid on, and hold in place with small bungee designed for affixing tarps or long small diameter coil spring

2. more secure...circular lid of aluminum sheet, 3 tabs 90/180/270 from each other with "J" lip (opening inward) that can catch the rim....4th tab is separate piece with a slotted hole to fit over a small bolt at the 360 degree location...snugged to the rim and locked in place with a wing nut