Ouch - sorry to hear you messed up a knee like that.

And given the hours people spend at computers, I still don't understand why most computer furniture doesn't have one of those foot rest bars as part of the design. To date, I think I've seen ONE built like that.

Something you might try -- get one of those single plastic file cases, or something about that height, and shove it under the desk to act like an ottoman, & throw a small pillow on top of it.