Here's my solution to the eternal problem of the lousy fuel caps on these things. I got one of these first.
https://www.amazon.com/dp/B01N5FSDDA?ref=ppx_yo2ov_dt_b_fed_asin_titleThe spud reasonably fits the hole in the tank. I secured it with JB WaterWeld putty, supposed to be gas resistant and suitable for tank repairs. It's the white stuff.
It cures quick and after it set overnight I painted a layer of regular JB Weld over the top. First trial it seems to do the trick, I guess I'll find out how well it holds up. So far no leaks and no seepage. I'll worry about making it pretty once I'm sure it will work. A dab of filler and a shot of paint should do the trick nicely.