Just a simple gas can cap insert. Mine are 6 gallon cans, but I'm sure this will work for a variety of sizes.

Took under half an hour to print. I made mine out of PLA but your situation might be different. I only need this to work once. If you need something for long term storage you might consider a more durable material.

• If you happen to still have your rubber gasket, it will fit over this.