Tips for bladder expression of a large paraplegic dog?

My dog has pulled his catheter out twice today. The second time, the vet said I could either reinsert the catheter or begin regular, manual expression of the bladder. He also told me how to express the bladder. The problem is that I have a large dog, and will be working alone. Diapers are out, because the incision from his surgery is still open. Does anyone have any tips for positioning the dog prior to expression? Or any other helpful suggestions?