For an upright post, the best I've found is linked below. Your cat will probably also enjoy the cardboard scratchers such as the scratch lounge. There is also a lounge with sloped sides.

My cat uses the post several times a day and enjoys sleeping in the scratch lounge.

Here is a link that might be useful: Post

I don't have an amputee but my cats LOVE the scratch lounge that annz mentions. They not only scratch but sleep on it--fight over it too.

I've had two tripod cats, one with a hind leg amputated I watched for over five years and one with a front leg amputated. I found that it is more difficult for the front-legged amputee to keep her claws tidy, even though she did scratch exercises. It simply meant that I had to check her out more often to see if she had nail issues. She wouldn't use any kind of post or flat on the floor scratching station, but she loved to rip up the sides of my record album collection. LOL.

We just got one of those pet fusion lounges for our kitty. So far he seems to like it. It looks good and seems to be a pretty nice quality. It seems like it might work for an amputee who wants to scratch- there are lots of angles to get at it. Not only does our guy stand or lie on top and claw it, he likes to lie next to it and reach in and under it.

After thinking about it, the scratch pad linked below may be easier for an amputee. The shape could be used to brace his chest while he sharpens his nails.

Here is a link that might be useful: Scratcher