Yes, I could cut the thumb hole but, as can be seen in the photo, there is a lip around the hole to keep it from tearing. I suspect they are using a heated oval punch to make the holes, but I don't know for sure.
Online, I have found quite a number of ID wallets with the thumb hole feature, and the cut outs all appear to be the same. This leads me to believe that either the makers are buying the plastic pre-punched, or there is a source for heated oval punches. If I could locate either the pre-punched windows or the correct oval punch, then I'd be all set.