Mower shops sell weedeater vent-caps that fit in the little fuel lines (1/8" I think, like for weedeaters).
I had a faulty gas tank on a brush mower, so I made my own gas tank from a stainless steel bike bottle, a 1/4" double barb coupler, the red handle-bar clip bike bottle holder, a little piece of weeder fuel line through the plastic bottle cap, and a vent cap pushed into the line. They might even give you one off a busted weedeater if they let you look in their trash.
Mine doesn't have the fuel shutoff because the original tank didn't have one. It works fine with the mower, and makes refueling easier because the gas can isn't near the engine when pouring.