dont see a point in just the rear half of a cage, its not going to protect where the driver is. If u want a real cage then get a "real" cage not just the rear, mine's a ex rockingham cage, so is f2 stu's and roly made his himself.
Speaking to ben16v (some of the older members may remember him) at eurofest, he had a safety devices cage in his, omp make one but its sort of show cage but a real cage, just not fia approved... look on ebay for it and they do a 10 point cage lol.
Hope u know a welder and dont fit it like the previous owner of mine and bolt it straight thru the floor.