I've used galvanised patio tube.
Obviously the base of the oven can be made of anything. All it does is hold your oven up at the right height. I'm not much of a brickie, and the straw bale option that I used last time was a bit too time consuming and messy. I find steel nice and easy to work with. Easy to cut and weld to just the right size and shape.
The top is 1500x1500mm square. This will be just the right size to fit the oven on and still give some room at the front for a smoke chamber and a small work area.
The legs are patio tube also. Set into the ground with rapid set concrete at about 400mm.
The top of the base will sit at about 1000mm from the patio decking. I've found that this is just the right height for me to be able to stand just back from the oven door and see into the back of the oven.
This base frame was made and set in place on the first weekend of the project.