I do not recommend going that way as they push the alternator out of its designed limits and leaves it open to destroy itself if the conditions are right, and those conditions will most likely occur when you out in a desert somewhere in the summer, very hot day and the batteries have been run down very low, result, alternator = toast.
Ill bet you that any that you buy will come with a disclaimer in an attempt to make you think they are not legally responsible for the damage they know it might cause. I thought about selling them myself, but would have to sell a hell of a lot of them to pay for the insurance policy.
However after saying that, if you do decide to go that way the commercially available ones are outrageously over priced for what they are. All you need is a silicone diode, and as it has to serve as a fuse as well, a very small one that will blow itself apart if it has to carry more than half an amp or so, most radio repair shops will have small glass diodes, then just fabricate a way to put it in in place of the fuse.