It's not easy to keep them from jumping after a surgery, but it sounds like you are on it.
The cardboard is a good idea. It works, then can be recycled.
My prayers are for him to have an easy surgery which takes care of the problem once and for all, a speedy and complete recovery, and for you to have an easy time with keeping him quiet for as long as needed.
Please let us know how it goes!
Love All Life, Thank You For Posting! :)