Here's the trick I used when I notched my rollcage: Go find a toilet paper tube, a razor, and a sharpe. Take the tube and start by cutting two very shallow U's on one side so it looks like a notched pipe. Start shallow then slowly take away more until you get a nice fit. Once your cardboard template is ready you do a different angle or pipe on the other end of the tube. Only do a template for tubes on the right or left side not both sides. For instance the rear cage stay, and a door down bar on the drivers side. The reason is, you can slice the tube long ways and turn it inside out which gives you a mirrored template for the passenger side of the car. Once you have a line down just use a grinder to take off material, work slowly and test fit often, patience is invaluable with cage work.
Here's a couple pics of what I'm talking about.
