You can do the junkyard crawl and grab a canister, vent valve, and purge solenoid from an older Ram or Caravan. The vent valve is the one way valve (it will have a filter as well). The fumes will accumulate in the canister. If you hook up the purge solenoid, one line goes to the canister and the other to engine vacuum. The solenoid is 12 volt, so hook it to a switch. When you start the car, flip the switch and purge the canister for a few minutes to clear it of the previously stored vapor.