There was a thread on this sort of frame that I posted last year— search Games with Frames. There are several photos of frames built with SpeedRail joints and with Cambridge Welding steel joints via Sid's Sports in SLC. Material specs and detail photos are there, if you read the whole thread.
Here's a shot of a frame for a small cat built this way.
For SpeedRail joints, go to <www.hollaender.com>. You can download a catalog and a pricelist. Minimum order $100. They use IPS (Iron Pipe Size) measurements that are confusing. The 1" IPS fittings are actually 1.315" OD and fit the smaller size chainlink toprail tubing (galvanized steel) that I use, mostly (shown in the pics). The 1-1/4 IPS SpeedRail fittings are 1.66" OD and fit the larger size chainlink toprail and
the schedule 40 (refers to wall thickness) aluminum tube that NRS uses for their frames. I've built custom add-ons for NRS frames using SpeedRail joints. I think the 1-1/2" IPS size fits the schedule 40 aluminum tube used by DRE, but I haven't worked with it.
There are tricks involved, the most important of which is to place dowels (I use wood) in the tubes where the fittings clamp down. Otherwise the tubing squashes and the fittings will work loose.
Anyhow, it works, and it's fun to build your own frame. You can also change it around as needed.