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We use 1/2 inch hardware cloth and critterfence. Our rabbits live in 2 small sheds in the backyard and have free range of the structures. They are not caged. If your rabbits are raised traditionally in cages suspended above ground, you may not have problems because rattlesnakes are not great climbers (they can climb, but not as well as some other snakes).
Rattlesnakes in our area can get through 1 inch openings. I really don’t know about 1/2 x 1 inch. It may be ok.
We live in an area that has rattlesnakes but rarely saw them until we got rabbits. We started seeing them much more frequently after getting rabbits. Our Rex doe was bitten by a rattlesnake in the leg but survived - that was back when we allowed her to graze on the lawn in a kennel with 1 inch openings. For the first 2 days we thought for sure she was going to die, but she had an amazing recovery and after a month you wouldn’t have known she had ever been bitten. Gosh, we’ve learned a lot this summer!