+1 on the wire (zip) ties. Lot's of uses for them. This was what I did on this summer's Great Lakes traverse:

I had the bike on the stand in Peterborough, ON and was evening the tension on my spokes when the rim gave way (Mavic XM 317, 26" 32 hole). Fortunately a piece of the rim still surrounded the spoke ferrule and the hole in the rim had a keyhole shape. I was able to wedge the broken section of the rim back into the slot on the keyhole and wire tire that spoke to its neighbor. It allowed some tensioning though not enough to completely get rid of a slight wobble.

I still had a day and a half ride to Kingston where I would complete the tour. It held up, fortunately. I suspect I could have finished the ride minus the spoke altogether but I felt better trying it this way.

Given the places I've taken it, I guess four years were too much for that rim.