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I currently have a reprap prusai2 from, NWreprap https://nwreprap.com/assembly/ the one on the right. Beneath the heat bed is a piece of plywood and on mine it has warped. I was wondering what would be a good replacement material for it that is heat resistant since some of the heat will transfer through the screw to it which is where the wood has warped the most. Thanks!
Plywood is by design dimensionally stable, it's rare that a properly maintained board would warp. You must have gotten it wet and then heated it, which is completely understandable given you have to clean the print bed with water from time to time.
The simplest replacement would be another piece of ply. I don't know what thickness it's supposed to be, but you can find the replacement part at your local home center, only for a few bucks (Make sure to get plywood, not OSB). It would behoove you to put a coat of wipe on polyurethane before mounting it. That will seal the wood from getting any more moisture in it and making it warp again. A couple coats on each end and face will do fine.
Two other materials you could try from the BORG (Big Old Retail Giant) are Medium density Fiber Board (MDF) or Melamine. They are both particle boards, but Melamine has a smoother, more water resistant surface. MDF, (Or HDF if you can find it), would need sealed with poly, and it takes a lot of coats since it likes to drink up sealant.
As for other materials, a block of aluminum would work, but that would require minor machining, and would act as a heat sink for the heated bed, causing the bed to take longer to heat up and cool, but would be more stable during the build. And it's not cheap.
HDPE is tempting, but is only rated to 80c for extended use. (http://www.stug.com.au/materials/eng...mperatures.php).
Looking at that link and at amazon, it seems you can get a 12" square sheet of 1/4" polypropylene for under $10. That would work well too, given it's got a continuous max operating temp of 130c and a short term max of 145c. Being plastic it should be easy to work, but go slow, as plastics can melt easy with standard tooling.