: 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.

If you have a concrete pad, like the floor of a garage for example, you can just bolt a frame to it - but if it's outside in a cold climate it may heave with frost.