Front carts seem to be problematic for some reason. Many such bipods learn to walk upright, like Faith. http://faiththedog.weebly.com/
If you still want to try a homemade cart, you can adapt any of the other homemade cart ideas. When Roo is of adult size, you will probably want to use a "real" commercial cart, if wheels work well for him.
I wonder if splinting or surgery might help straighten his left arm? I'm not sure how functional it will be as bent as that. Many cats, including one of mine, straightened their bends as they grew, but it's hard, if not impossible, to say whether bends will straighten or bend worse as they grow.