Spiral rib socks
are supposed to be really comfortable and easy to make. That link doesn't have a photo, but this picture
gives you an idea of what they look like. You can find more patterns if you do a search for "spiral rib socks." I just chose the first one that came up on google. The basic idea is that you make a ribbed tube, but shift the rib every few rounds to make a spiral pattern. This is supposed to make the sock fit around heels and stay up even though it doesn't have a real built-in heel. An improved tube sock, really. If you need to make the upper part wider to go around the cast, you could probably use larger needles just for the top portion.