Finally got around to welding on a washer. So far so good but I'm not really happy with it.
First lesson learned. There are no readily available commercial washers that are a good fit on a 10mm bolt. 3/8 is tighter on it than 10mm.
9mm won't fit over it, nor will 5/16.Second lesson learned. The hole is worn out oversized on both ends. There's only access to weld a washer on the front. The back hole is still oversized and the washer fix won't work.
Third lesson learned, the thickest washer Fastenal had in stock is still much thinner than the part being repaired.
Fourth lesson learned. The Harbor Freight 80 amp inverter welder is completely adequate for this repair--it's quite capable of slagging down the whole washer if you get careless with it.
Fifth lesson learned. The hole is worn out enough that after the weld it may be necessary to adjust the position of the axle to get the bolt back in.
Sixth lesson learned. A Home Depot Workforce 16 ft x 1-1/4 ratchet tie down does a fine job of adjusting the axle position.
Future action. When the weather warms up I'm going to jack it up, weld both holes around the rim and redrill for a good fit on the bolt. That's really the "right" fix.