I once 'cloned my cock' to give to my girlfriend shortly before I started work away from home.
Forget exactly which kit it was, but it involved using plaster of paris (for the casting process) instead of the funny rubbery stuff some provide.
Relevant points:
1. Maintaining wood for the length of time needed is harder than you might think - especially if you're doing it on your own with a view to making it a surprise. I imagine it'd be easier if someone was there to tickle your nipples or something.
2. These kits are designed for the average sized tallywhacker. I don't consider myself "huge" (7.5x5.5) but the model I opted for, did not allow me to fill the cast I had made - not enough latex, hence my fake dong is only about 6.5x5.5. Would recommend buying two kits if you've got a whopper.
That said, really quite impressed with the results. And so is the Mrs