Thank you! It was very much a room for beginners since that was most of my friend group but we had fun.

The disc was a child safety lock! You get four of them in a fairly affordable pack and the magnet could be hidden in something like a statue or chess piece, etc. They install really easily.

Child Safety Magnetic Cabinet Locks - vmaisi 4 Pack Adhesive Baby Proofing Cabinets & Drawers Latches https://www.amazon.com/dp/B07919W2C4/ref=cm_sw_r_cp_api_i_E2J023HM3K2VKQ83SB72?_encoding=UTF8&psc=1

Lots of magnetic locks on the market. Here's a cheap example that would work if you mount it at the top inside of the door jamb

You can get better quality if you spend more, just look around a bit.