I've built a couple cages for Club and Pro level rally cars. SCCA and Now Nasa/RA
Lucky for you I have Goumba's X-RAT Sitting here to use as a nice movable Jig. Actually that car is getting caged too Hopefully I'll get him down here and we can do both cars at once. It's easier to build 2 of everything at the same time.
Price is $180 Per Point. 6-8-10 or more.
There are some Pre-homologated FIA cages. all this does is avoid inspection at scruitneering. but they typically sell for 1400 brittish pounds add shipping and it's way cost prohibitive over building.