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I have also had much success using rubber "O" rings. It took a few trys to get the right size. You need a tool to place them. I used a cap from a drawing pen. You push the o-ring onto the tapered end and slide it near the open end. Then place the cap over your nipple. Then roll the rind off the cap, around your nipple.

On clamps and string...
Practice finding the length to use so that a clamp on one nipple, is on a string, over an overhead bar (closet bar), down to the other nipple, such that you must be on your toes. Then place your handcuff keys on a table nearby. Put the clamps on as above, and cuff your hands behind you.

Chemical heat
Try wintergreen oil. You may want to dilute it with hand lotion.

Mouse and rat traps
You can use mouse or rat traps as clamps. But they don't feel as strong as you might think. But if you are into photos, they look better that they feel.

Painting...
You can make your nipples erect anyway you wisg then apply several coats of nail enamel/ nail polish. It is not painful or irritating. But worn all day with your nipples held erect can get itchy and annoying.

Ice...
You covered molten wax. You can also get results with ice. You can have a victi... eh, subject in bondage. Show him/her some metal object held over a fire. Blindfold him/her. While rubbing their nipples talk about them receiving your brand and how the object is glowing. Excuse your self to get better gloves when you actually get an identical object from the freezer. Touch a nipple with the cold object. It will feel hot to them.

Sandpaper.
You can increase sensitivity using fine or extra fine sandpaper. Don't over do it.
...

I am sure that others will have more safe tortures and irritations.