How to stiffen 1/2" aluminum tube?

[Myka]

I made some aluminum arms to hang my ATI fixture. I used aluminum and brushed it to match the fixture. They look really good. However, my (cheap) tube bender only goes up to 5/8", but I couldn't find 5/8" aluminum tube, so I bought 1/2". As expected it's fairly flimsy/bendy. Any ideas how to stiffen it up? I could fill the tube with something - not sure what.