The other day I was wandering through my local hardware store and I spotted a small clip and rope package that looks like it will be just the ticket to tie odd shaped things to my rack.
Amazon has them here:
Small Figure Carabiner 2 pack
It's basically just two clips and about 8 ft of 1/8th diameter cord on each clip. That's enough to clip to the rack, tie an end to the rack, and then loop the cord around everything I need to hold in place, and back to the clip and cinch it tight. Lighter than a bungie and more secure. More work, so I'll probably still bring along a bungie or two, but sometimes you have something that just won't sit in the right place, like a replacement rim and this is just the tool to have with you.
Doubles as a tarp holder if you are out camping as well.