Stir plate lid

I got my hands on a hot coco warmer everything works except the warmer, it has a 1 gallon stainless pot with spigot and sealed magnet that spins the only problem is no lid. So how can I seal the top to use it as a stir plate? It is 8" in dia. Any ideas?
Thanks randy

Sanitized foil. I used a coffee pot with a 6" opening before I got a flask. :cheers:

This is what I would use.