Well instead of coming up with some sort of mount or plumbing a line to an external tank, I found a cheap option. About $15 for the frame bag on Amazon. lol
2 liter full of gas fits perfectly. As well as my U lock, cable lock, tube, lever, etc. I know people advise against using a plastic bottle. But I know people who have stored gas in them for years with no issues.
I didn't want a 1 gallon jug on the side or anything to keep a clean look. So bag worked out well. And it's thin. Figured 2 liters is just over half a gallon, so should be more than I need for my 20 mile rides.