aluminum vs. steel tubing
In the 'Similar Threads' box below there's a listing for 'Games with Frames' where I posted quite a lot of info (and some photos) on fabricating your own custom cat frames.
I use galvanized steel tubing (intended for chainlink fencing and called top-rail) that's 1.315-inch OD, which is as strong as the larger alu tubing in NRS frames while being about the same weight per foot. The corners and joints are Hollaender SpeedRail aluminum fittings, with the 1-inch size being right for the top-rail tubing. I've also used conduit ells, drilled and bolted, for corners.
The major design factor is to "box" your frame, i.e. have at least two tubing members in every direction where flexing might be a problem.
The two details I'd rate as most important:
a) drive plugs (hardwood dowel is good) 2 inches long at the ends of tubes so the setscrews in the joints won't squash them (also to keep water out).
b) File and sand off all the sharp edges— a rotary tubing cutter leaves nasty razor curves (drill holes likewise). Also, make sure none of the setscrews in the joints will bear against a tube.
The nice thing is that you can build a decent frame for around $200-300, and tweak it to perfection. It breaks down for winter storage or bush-plane transport (also for long-distance travel inside the vehicle).
If you're going to haul big loads on big water, say an 18-ft. cat with 28-inch tubes for a Grand trip, get a component-style frame from DRE or NRS. If you're running a sub-16 footer on III and IV water, a homebuilt frame will do fine.