The pictures in the thread should show how the whole thing goes together.
Attach a 4" length into the hub and attach the side of the final 90* tee fitting.
Build the entire thing and tweak the shape so it runs smoothly.
cross-drill and attach machine screws at the joints where indicated:
The screws should run as shown in the drawing to help stiffen the frame against sagging. Mark each joint with a sharpie so you don't confuse which section goes where when you re-assemble it at the range; otherwise it won't bolt together again.
Duct-tape the binder clips to the end of the gardening stakes and you're all set to go.
Anything that gets shot is easily replaceable.
Any additional questions or anything I've left unclear, just PM me.