I used a 3 x 3" square steel tube. Worked great, rigid, but very heavy.
Aluminum would be better but lots more money.
If you're using round tubing, you might have some problems aligning your forms.
If you are using a round tube, then running a strip of wood along the top, would solve any sliding around.
You could even use thin wall tubing, by ripping a 2x or something to slip inside. A few screws and your tube would not only be
rigid but straight.

Don't forget Nick's 'spacers' I found this part to work really well.
As soon as you add a couple of strips the whole set-up becomes nice and stiff.