My family started playing Ticket to Ride. I routinely forget to complete one of my destination tickets because it is the back of my hand.
I purchased the TTR Asia game to get the card holders. Functional but I wanted more. I don't know what the perfect size or number is.
I am listing several I have worked up. The above are from my fathers stash of lumber. The Six are  rough sawn  oak. The oak is very heavy. All are about 6 3/8 inches long. They have been sanded three times, and three coats of finish have been applied. I have simple tools so they are far from perfect but should function well. If you need more or larger look at the other postings. I will combine shipping for $2.00. I have included an Asia card holder for comparison purposes.