A good cheap backup for jack stands would be nailing a few pieces of 2x6 together until you get enough height that if the car drops off the stands it won't crush you. It's light, stable and stackable as long as you make the cuts long enough.
I glued (PL Premium Plus) and screwed (#8- 2 1/2 deck screws) cut off 2x12 stacked lumber to appropriate height for secondary back up. Put drawer handles on for easy carry. Working under car on back gives willies, one of half dozen reasons my hot rod fantasies shall remain fantasies. Will never shake Dennis Gages' hand.