I do this all the time. I fixed one pipe last night and have another in the freezer now.
To block the big hole I have an exhaust coupler with a 1/2" socket extension zip tied in it. The steel will help the water freeze faster and seal the gap. My friend who taught me about the steel actually has a section of steel rod that is the correct diameter to fit in the opening but I made do with what I had. Then I push the open end of the coupler on the pipe and zip tie it tight. For the stinger I block it with an old bent pivot ball and electrical tape is the ticket for sticking while it is still wet.
The process kind of goes like this:
1) Pull off a 1-2' long piece of electrical tape and stick on counter.
2) Zip tie blocked exhaust coupler to pipe
3) Fill sink with water
4) Submerge pipe and fill with water (critical point #1 is to shake it around to get all air bubbles out)
5) With pipe still submerged, put pivot ball in stinger facing upward.
6) In left hand, put thumb over pressure nipple and lift straight out of water with stinger facing up. (critical point #2 is do not lose any water)
7) Grab electrical tape in right hand and wrap around the diameter of the pipe and pivot ball slowly but tight while not losing water.
8) Gently place in freezer at that same angle. Make sure you have something in your freezer to prop the pipe up so the stinger is facing straight up. I use a couple of old rubber sedan tires
9) Let freeze for 4-6 hours. If there are still dents, thaw in warm water and repeat 1-8. I had one pipe that took 6 times but it finally looked and worked great.
I thought about the zip ties around the pipe to fight over expansion but my fear is I'd end up with a turbo ring looking pipe.
WARNING: If there is any hole in your pipe including a pinhole in the weld, this with likely destroy the pipe and blow it open.
If anybody doesn't understand and sees me at the track, ask me because I have these pieces in my toolbox and I can show you.