Buy or Build?
I'm a stellar fan of Jack's cat tubes: we've got four sets and despite all manner of hairball runs over almost twenty years have never had a puncture. Tough, and beautiful.
I build cat frames (for my boats, not for sale). It's fun and costs 1/4 - 1/3 what you'll pay for a DRE or NRS cat frame (both excellent and highly recommended).
If you look at the 'Similar Threads' box below the last post on this thread, you'll see 'Games with Frames'. Scroll through and you'll find specs for homebuilding and sources for materials. On p.2 you'll find photos of a homebuilt frame for a 15 ft. Jack's Flyer Cat with 19" tubes. It's been down our local runs hundreds of times, Deso (high water), Lodore, and Northgate Canyon on the North Platte, with no problem whatsoever.
Using galvanized chainlink toprail tubing and SpeedRail joints, I've only had one bollix and that was an ultralight frame with only one front cross tube (both footbar and frame member- risky). One end torqued loose in a hole, but I pulled into an eddy and fixed it in a jiff. Twisting loads are a problem. I rebuilt the frame with two front crossmembers.
On the Grand Canyon, the combination of heavy loads and many rapids can be tough on frames. So whatever you choose, check the joints and especially the straps that hold the frame to the tubes. A loose strap can break under a shock load and cause other straps to fail.
Jack himself had a bunch of straps blow on Lava Falls— he was rowing his sister's Daddy Cat through and some straps were evidently loose. Spooky. It's easy to pull up to camp and unload and start the booze-up without checking your rig. But a quick go-round with a wrench and a jerk on every crucial strap doesn't take much time, and you'll sleep easier.
In any event, good luck—