Stay away from PVC or other vinyl if the chip will contact the pages as they will bleed reds from many Chipco chips.

I use 2 1/2 x 2 1/2 mylar cardboard flips. Mylar is neutral and won't interact chemically with the chip. Many people use 2 x 2 flips, but I think there's more room for writing on the 2 1/2 x 2 1/2.