How to Make Your Own Copper Pipe Garden Ornaments
Page 150 of the Trellis Craft book mentions using 1/2 inch rebar as an internal support for 1/2 inch copper pipe projects.
Some book owners reported that they cannot get 1/2" rebar to fit inside 1/2" copper pipe, so I went back to the home centers to double-check the rebar, and found these problems:
When going to buy rebar, it is a good idea to take a short section of 1/2" pipe with you. You can then use the pipe to check for any of the above problems before you buy the rebar.
The easiest and cheapest solution is to use 3/8" inch rebar inside 1/2" copper pipe. The same back-and-forth bending that is a problem with 1/2" pipe actually helps 3/8" pipe fit snugly inside 1/2" pipe.
Try to find pieces of cut 3/8" rebar that are straight overall. Then sight down the length of the rebar from one end and you will probably see that the rebar actually bends back and forth a bit. These bends will keep it in contact with the walls of 1/2" pipe and make a reasonably snug fit.
I have tested this by making several pot trellises from 1/2" pipe that are supported internally by 3/8" rebar. The end result is good support. There are some step-by-step instructions and photos for making these pot trellis supports here.
If you want to support freestanding trellises in your yard, it would probably be a good idea to use 3/8" rebar inside each leg of the trellis.