You can use whatever dimensions you want, but mine are: 2@ 12.75", 4@ 5", 2@10" (the last two are the upright pieces--and not in this picture). This configuration gives me a roughly 18"x18" base.
Glue the 90s onto the 12.75" pieces, and use a level surface to level the 90s before the glue sets. The purple stuff is primer, and the glue is clear. Wear safety glasses and gloves when doing this. PVC primer and glue are seriously caustic.
Assemble the pieces thusly, and use something to level the tees. Being a plumbing apprentice, I have a torpedo level to do this.
|You didn't build that.|
This is what the completed stand looks like. This is also what the inside of my garage looks like. If it gets windy, you can stabilize it with a sandbag, or get really creative and fill the PVC with sand. I don't hold the furring strips in the PVC with anything but gravity. If you shoot one off, you can pull it and replace it quickly. Enjoy your new target stand.