Not really my first choice in external tanks, but I know their less expensive than a steel one.
All gas cans are supposed to vent, it is dangerous to not let them vent. Most (even your car) are vented thru the gas cap.The internal tank on my boat has a seperate vent because I installed a watertight cap.
As far as the spillage goes, have you checked the neoprene or cork gasket in your cap to see if it is worn or missing?