This was a fairly simple desk design. I used a pre-finished 48" shelf and some wood blocks as risers. I then added stopper blocks on each end of the risers to keep the desk from shifting or sliding off the treadmill arms.
All the blocking is screwed together and the desk just lifts off if I want to use the treadmill for running. No permanent attachments or alterations to the treadmill.
I had her walk comfortably on it and measured where her foot came off the tread compared to the end of the tread and she still has 15"-20" before she would start to slip off the end. Still plenty of usable tread left.
The finished product and the risers/stopper blocks:
