I have drilled & filed a large hole where i want the fill neck to be located on the tank. The hole should be sized for a 3/4" pipe to slide thru nice and tight.
Then i cut a 3/4" steel merchant pipe coupler in half with a hack saw and braze the cut off piece to the inside of the tank, mig welder would probably work also. Make sure that you line it up pretty close.
Then screw a brass 3/4" male pipe thread x male garden hose adapter thru the tank hole, into the pipe coupler. Use pipe thread tape on the adapter to keep it from leaking. Tighten it up till the flange on the adapter bottoms out against the tank.
Use a brass garden hose cap for the gas tank cap. You will have to experiment with the rubber gasket. Some will resist gasoline, some will not.
Everything should available at a good size hardware store. Makes for an easy filler & looks good.