I don't fancy welding a washer onto an original type.
Ah no welding required, the washer slips over the shank (inside toolbox cover) it's softish and dished so once on the shank and over the groove in same you just slip a tube or socket against it and push it flat, this squeezes it into the grove and makes it captive.
I also fitted a very thin washer between the screw head and outside of box cover so turning the screw head doesn't rub off the paint
This is on my Flash (fairly original), my other bike is not so original and that is the one where I have a ring on the fixing, I'll try to find out where I got it