This is a little surprising to me...

When I first joined this forum, I didn't know where I could get a 2x3 Fresnel. So I did exactly what is being suggested here, I bought the bigger sheet from a vendor on eBay and then cut them to size.

I didn't seem to have a problem. The cards cut from the corners are off-axis in their focus (you have to hold them at about a 10 degree angle to your target), but they certainly seem to work.

I can't imagine my cutting technique was a lot different that the rest of you (I used scissors). I even radiused the corners so they wouldn't be sharp. I've got one in my wallet right now.