i was working for a company called national parts... we supplied to the place todd was working at the time... it also meant we could do the fire extinguisher and bonnet strut group buys... fire extinguisher prices turned out to be nothing special, as you could get them from big W for not much more... (buying power of a supermarket trumps almost any player in the automotive industry), but the bonnet strut deal was a pretty good one, we dealt with gabriels quite heavily, and imported 1 box only of the supra bonnet struts, they were spoken for as soon as i announced we were going to do them, even though i warned everyone they'd probably take about 3 months to get here... turned out they did, and all the original people still wanted them....
not there anymore cos the company went bust (combination of some poor management decisions in the distant past / GE credit pulling the pin, actually before the 'global credit crunch' really hit / other states not meeting their performance targets - WA was performing really well, but we were the only state really pulling their weight, and it finally hit us)
now working for a holden dealer in spare parts, dealing with many of the same parts, buying off people we used to sell to... to start with it was really frustrating knowing how badly we were getting screwed on parts, but i'm getting used to it now...
anyway... back to the cage... the reason i suggested talking to a builder, is that i'm pretty sure you'll need someone suitably qualified to sign off on it... and they usually won't sign off on anyones work except their own... as if you have an accident, and the cage fails, it could come back on them...