I haven't found any way of getting around holding my son down during blood work. I can't count how many times we've had it done and I've had to hold him down everytime. I'm always right there trying to talk to him and comfort him though.
Also, stool and urine are both pretty easy. Urine is especially easier on a boy.What I've found works best is to tape the bag on and then put sweatpants or something loose on without a diaper. Everytime I've tried to do it with a diaper I never end up getting any urine in the bag. Also, with stool they typically give you everything you need & we've always gotten something to scrape it off with.
Also, stool and urine are both pretty easy. Urine is especially easier on a boy.