I made my own cork gasket when I couldn't get one a few years back and coated both sides and the screw threads with Threebond 1104.
Fuel resistant, and much thinner than any RTV etc they make. Similar to Stag.
Here's one example:
NEW Threebond 100G Gasket Adhesive Grey 1104 Best Japanese Products | eBay
Works a treat, doesn't go off in the tube either, my tube is 8 years old now.