I'd be willing to help with this. Looks like something we could bang up with some basic welding & woodwork techniques combined with sewing / leatherwork. I'm better at the first two than the later, so hopefully you can cover those.
The "above the knee" design would be tricky to manage. Designs I've seen don't do that, but it does seem a good idea for long term use. Probably the best option there would be to look for some sort of commercial product that can do the job of the joint (either a commercial leg brace or some suitable pivot) and work down from there. The picture looks to me like they started with a commercial leg / knee brace, which is a good move, but expensive. You might be able to pick one up used for cheap, but finding a matched set like that would be hard (though not 100% needed, as long as you have a left and a right).
Below the knee designs are easier to make, but harder to walk in, and (duh) potentially cause knee pain.
Of note is that I can't really commit to helping with any project builds until I finish my custom motorcycle project. Its easily got 100+ hours to go still, and is due to enter a contest in early May.