Hi and welcome. Thanks for registering as a member, your future posts won't need approval so post away. What's your kitty's name? 

Why do you think your new kitten needs a prosthesis? Is she having trouble getting around? Animals typically do fine without one, but  a well-made prosthesis can be extremely helpful. They can give the animal a normal gait and help avoid issues that arise because of a compromised gait, like arthritis and joint stress.

Unfortunately, there are two reasons why a prosthesis doesn't work in many animals. If there's not enough stump left to attach it to, it just won't stay on. Basically you need to have a lot of leg left below the elbow in order for it to work well. Not to say that it can't be done, anything is possible, but that is the ideal situation.

Also, we have been told by veterinary rehab therapists that homemade, DIY and most of the 3D printed prosthetics people make will do more harm than good in the long run. That's because unless the designer has a solid education in animal "biomechanics" (i.e., the way an animal's body is designed to move when walking, running, jumping, etc), if a prosthesis is even a smidgen too high, low, big, small, whatever, it will throw off the animal's gait even more and cause more trouble than without one. We know this is true because we've seen many folks here use non-custom orthotic devices like leg braces, and when they didn't fit well, they caused blistering and pressure sores. The same thing happens with homemade prosthetics . 

Here's an article that tells more:

Facts About 3-D Printing for Pet Prosthetics

Most cats do great on three legs. Just check out our TriKitty Bloggers:

Tripawd Kitty Bloggers Sharing Their Feline Amputation Tales

Now, the best thing you can do for the kitty is to focus on helping her get strong so she can maintain her balance. You can do lots of fun activities with her! These articles will give you some ideas:

Tips for Three-legged Cats During and After Amputation Recovery

I hope this helps! Be sure to post some photos of your kitten, we'd love to see them. Here's a post about adding images to the Forums.