What we mean by a onesie, are the little one-piece outfits you put on infants (human babies
), with the snaps between the legs -- that keeps the puppy from chewing or scratching the stitches. I think that is a great idea -- wish I had had it when we did Libby!
If you go with a cone or collar, what we found was that, if we were around, we could pretty much keep her from chewing without using the collar. But, if we were busy, or out of the room, we would have to put the collar on. She hated it!
In our case (of course, Libby was younger), Libby was up and ready to play (even though she was still in pain) on the next day. The vet was not happy about that and, when she had a reaction to the sutures, she had to be sedated--at home--for two more days. I am not a big fan of pain medication/sedation, so this was hard for me, but it really did make it possible for her to heal. She was just so full of puppi-ness. Again, since you are dealing with an older one, you might not have that problem.
Careful about feeding with pain meds, or you'll be cleaning up spit up.