I make all of my card slots the same size. I have an acrylic template that I use for the T slots. It makes cutting the slot a whole lot easier because you just cut around the template instead of tracing around another thinner pattern, and it also seems that the t slots are all the exact.same size. I generally will use somthing like a thinner piece of leather for the slots like sheepskin. When I first stated making wallets I had the same problem with the sizing being off. I found that my main issue was in gluing the slot material to the primary leather piece. I just learned (the hard way) to be very careful in where I put the glue. The other issue that I had was that I did not skive the bottom edges of the card slots and the thickness of the leather would throw off my width.
