For my heavy 10 underdrive, I went to the local Fleet Farm and bought the cheapest serpentine longer than the original- under 20 bucks. I cut it and spliced it by using a V-shaped butt joint and a piece of the extra reinforced backing sewn over the joint. I also tried using a "wonder glue" to keep things flat but it didn't help- RTV silicone based. Maybe a urethane-type "Gorilla Glue" would work better.
I used a braided poly cord as thread and drilled the holes. I left the grooved side in and put the thread in the grooves.
How does it work? Great except my knots hit the pulley cover on the lowest speed pulleys. Also, these don't stretch like leather so you need to run less tension. My lathe came with a woven poly belt with and alligator splice. It slipped easily and needed a lot of tension. Probably explains why the lathe's countershaft was broken.
In the future, I plan to grind the splice a bit thinner and move the knots to the sides. I hated the sound of the alligator clip so this is better. I run the lowest speed with the top cover open.
Hope this helps,
Bill