I got from Amazon one of these:
Xinnyuan Outdoor Copper Quick Inflation Butane Tank Connector Cylinder Coupler Gas Refill Adapter Pneumatic Valve – $3.85
that brand will probably be out of business but there are other brands. This must be from China so with the trade war, maybe they’ll raise the price or they’ll quit selling them totally.

That worked pretty good, I was surprised. It’s one piece of metal and one o-ring. Nothing to fail.
In order to get all the fuel out of the cartridge, don’t hold it vertical like in that picture, hold the cartridge almost horizontal, slightly downhill, with the slot on the cartridge down. Put canister in freezer first, and cartridge in 110F water. Know the gross weight of the canister (7 ounces for 4 ounce size, 13 ounces for 8 ounce size). Weigh the canister after you fill it and don’t let it weigh more than that limit or it can self destruct.
8 ounces of butane from Korean grocer costs $1.25. Package of 4 cartridges for $5. What does an 8 ounce canister cost, $6? This device pays for itself with the first cartridge.
By refilling, you can put just the right amount of fuel in for a trip. You don’t have to take a full canister if you don’t need that much. If you have a mostly empty canister you don’t have to take a second spare canister.
The cheap butane is good when the air temperature is above 32 F or so. Below that you have to use something like a Moulder strip. Good butane works at about 11 F colder.

