I have a 4-liter Happy Time peanut tank mounted on a rack behind me. It has served me well for years, having been mounted on the top tube and the rear rack. It has a locking gas cap with hinged lid which vents the fuel system. The only problem is that the fuel sloshes and finds its way out the keyhole. Sometimes at the end of the ride, the lid is up, but that's not a big issue. I ride strictly on paved roads. Does anyone have a simple inexpensive solution to prevent gas sloshing up towards the cap?