I made this simple tool years ago as a "third hand" when doing various welding jobs. I've found it very useful for autobody work. Your torso seems way more stable and easy to control the pressure than other methods, you'd be surprised how easy it is to use, seems to come natural for some reason.
Simple half inch square tube T handle with a solid rod inserted and a simple device to cinch down on the rod. The rod slides in and out to adjust the length. You put the T side on your shoulder, or chest and adjust the rod and use it to press things into alignment for tacking. Can also be wedged into areas to hold something in place, simple device that comes in very handy for all kinds of things.
I'm sure others have built the same thing, and someone probably manufactures them too, never bothered to check.