i actually do have a part number for a rubber/plastic cap(non vented) from Napa.
its a oil filler cap and you can make a hole in center and put a tube for venting.
part# 703-2300
it fits the tank but mine flew off when i hit a bump but i may get another one and try it again since it doesnt leak. and it costs only $2-3

maybe this time i will add a strap on it so i dont loose it,lol.