I vent my plastic cans with a schrader valve like you you use for air etc. Drill the hole a little smaller and screw it into the can with a little silicon. Remove the valve core and put the cap on it. No leaks. Remove the cap when you need to vent. My main problem is misplacing the cap so I keep extras on hand.
This made me recall using a potato for a stopper on kerosene or coal oil cans when I was a kid. Seems the metal screw-on cap got lost soon after purchasing a can.I fitted schrader valves to a couple of 5 gal fuel jugs. Both broke off in less than a month. The rubber "groove" broke at the base. I did not expect that, I guess I'm hard on them.
So I wittled a couple of wooden plugs, and tye the pegs to the jug handle with string so they don't get lost.
With temperature change, I have had the wooden plugs loosen, but it's easy enough to check them before handling the jug.
Also, where does it say a 5G can must be filled with 5G, I put 4 in mine, makes pouring into small tanks much easier. I simply bought a few more fuel cans, right now I have 15 fuel cans; 1 kerosene, 3 diesel, and 4 gasoline, with 7 unused in reserve. Oh, 3 of those in reserve are safety flame proof cans; I dislike them but got a great buy about 30 yrs ago. When we camped in a trailer and carried a portable genset I did carry them in the truckbed.See post #125 in this thread, where they said they dislike lifting 5 gallon cans.
It's not so much the total weight (5 gal diesel = 35.5 lbs). It's the awkward position they have to be in to fill some of my equipment - combine that with the slow pour from crappy modern nozzles which means staying in that awkward position for more time and I can see why some are looking for other solutions. between a nagging upper back injury and a left shoulder that occasionally has problems, I suspect it won't be that many more years before I'm partially filling my 5 gallon cans.
They make a rule, then just let industry figure out how to abide by it. How long have they been trying to figure out how to make a gas can? 20 years?Steve, that's what happens when you have morons making the rules.