Just thought I should show you a picture of my knitting rig:
One end slides under my leg, to hold it. The mechanism basically just holds the needles, as you can see in this picture. The white conical shaped piece is the collet that holds the needle in place. There is a different sized collet for each size needle; I have a collet for sizes 0, 1, 2, 3 , 4, 5, and 6. I'd like to get more collets for sizes 7, 8, 9, 10, and 10 1/2, but I can't really afford them right now. It is highly adjustable, too: you can raise and lower it, and adjust the exact position of the arm and the angle of the collet. The device is entirely hand made by my occupational therapist's husband. I think it's a pretty clever way to knit with four needles!