Yeah when you do this for a living the expensive tools become life saves in the end. The PM’s are nothing new and do work for the most part, but having been building cages for over 15 years now you just learn faster ways to get the job done than dick around with some finiky pin-type markers.
Lolz’s at the grinders/cutoffwheel method. The joints are only as strong as the fitment of the material. Get a coping machine, The cheap ones are not that expensive and will suit this just fine.
You need a coping cutter/notching tool Something like this from northern tool is a very cheap but effective machine to get the job done. runs off a drill press on the table, or clamped in a mounted vise and large 1/2" electric drill to drive it… You’ll get 3-4 10pt cages from one of these before the bushings shits the bed and the saw starts to wander. There are other more expensive versions with needle bearings that are quite nice, but far from a production machine.
http://www.northerntool.com/shop/tools/product_23000_23000
~$100
Harbor Freight also has a version of above for $60.
I have a massive machine I built to handle the demands of full time rollcage and tube chassis production. 2hp motor, 4:1 reduction. It’s a beast. I copied the design of Baliegh Industrial, only made it three times as large to handle up to 4" tubing. About 6’ long and weighs about 350lbs. Their machine picture below is about $2500. I built mine from machine spares at the machine shop and steel. Would retail around $4K if new, but again when into this stuff good machines don’t come cheap.