Also my current method, I've noticed the truck around as I go about my business, just stop and get one every few weeks.
I had to check this as I have 2 20mm gearwrench, neither of mine is reversible which I believe is what you were referring to and it seems that there isn't a 20mm reversible. Very odd to my way of thinking but on the other hand I've rarely had to deal with the random 20mm.
Reversible or flex head. You have a zero offset ratcheting combination?