I have the exact same cans as the OP and the exact same issue. Love the cans, but holding that button down kills your thumb. While I applaud your ingenuity on that, I just drilled a hole across through the 2 yellow tabs, right about where you mounted those screws to it, and stick a 3/16" drill bit through it to keep the button pressed down. Push the button down all the way - line up a spot to run the drill through, and poke the holes. Give it a little slack so it doesn't bind up. A whole lot easier than your method, as long as you have a drill bit handy ( I always fuel up in my shop, so drill bits always handy). I suppose i could tie a piece of steel rod to the can if I really wanted to keep it there all the time...
Only risk is overflowing if you misjudge how much space is in your tank, or a flood of diesel if the can gets knocked over or falls while pouring. Mine stays put nicely, so I can do other things while it fills. Though I always stay close just in case.