I've built nine frames with locally-available and cheap chainlink toprail galvanized steel tubing, 1' IPS, 1.315" OD, with fittings from Hollaender SpeedRail or Diamond Aluminum. Per foot, it's lighter than sch. 40 aluminum, but not as stiff, and the thin walls can be crushed by setscrew fittings. So I plug the ends with 1-1/4" hardwood dowel set in epoxy.
This is a research cat built for the University of Wyoming:
I've been running these frames in class III and IV water for years with no material failures.
Also built an ultralight cat frame of 3/4"IPS galvanized steel pipe with sch. 80 aluminum siderails.
Details under More Games With Frames