I haven't found anything more durable or convenient in the eleven years I've been building them. Mostly because it doesn't need to be more durable.
A clothespin is just a lever with a built-in return spring, so you can use pretty much anything that serves the same function, and any kind of spring. All the clothespin has to do is lift the nail, and then return it after firing. Keep in mind though, that the clothespin should have pretty much zero stress on it during firing and cocking. All the work of resisting the spring is done by the nail and the PVC wall.
In most every case I've seen where people complain of a "wimpy clothespin", it's been because the nailhole was drilled too big, transferring far too much motion and stress to the clothespin, causing binding. The SNAP has its share of faults, but the clothespin being too weak for its necessary function isn't one of them.
Hello. I am Indigo of the Rainbow Clan. You Nerfed my father. Prepare to die.