Take along a 120 spool to the hobby store. Find the size of K&S nesting brass tube that exactly fits in the center hole, its inside diameter will fit nicely on the 620 spool pins. Buy a K&S tubing cutter while you're there, you still won't spend $10 and you can cut the 1/4" lengths you need, cleanly and squarely. Assuming you take up to original 620 spools, you won't need to do any more than slip a pair of the bushings into the trimmed 120 supply spool when you load, but if you don't have enough 620 spools and the camera can handle the extra 0.1 inch of length, you could also slot the bushings to fit the 620 takeup key and epoxy or super glue them into the trimmed 120 spools, to use the same spools for takeup. Done assembly line fashion, this could be little enough effort to make those 620 cameras even more practical shooters than they are with respooling or 620 spools (since you can make as many as you want/need, you don't have to worry about getting the adapted spools back from the processor when you do color).

Alternately, if you're handy with a soldering iron, you could get a suitable piece of brass C-channel while you're at the hobby shop, and solder it to the bushings to make keyed adapters that could be removed from the takeup spool when unloaded from the camera and reused indefinitely. And if someone were to make a bunch of these and sell them (perhaps stamped from aluminum or steel, wouldn't have to be brass), I'd buy a bunch if the price was reasonable.

Sadly, of my current three 620 cameras, only one will accept even a trimmed 120 supply spool, the others can't handle the length even though they'd be fine with the diameter.