I hope someone answers this cuz I've been wanting to know the same thing. Once I saw someone slip something flexible inside copper to keep it from crimping when bending, and for the life of me can't remember just what it was.

There are two methods you could use that will work with reasonably small radius bends.

One is to cap one end of the section you are working on and, using a funnel, fill the pipe with play sand, packing it down with a rod as you go. Once full you cap the other end and you can bend away.

For caps you can look for pressure test plugs or use a compression fitting and threaded cap that can be removed after you do your bending.

Soldered on caps would also work fine if you oversize the piece then cut it to final length after bending.

If you have cold enough weather you could also cap one end and fill it with water. Leave it upright outdoors overnight and it will freeze solid giving the same result as sand packing. Just be sure not to cap both ends as the water will burst the pipe just as in an unheated house. And use gloves when handling the frozen pipe.

The easiest way is to use a pipe bender.

You can buy one pretty reasonabley for something that small, or you can also rent them.

They are not hard to learn to use.

Janie