Mighty, I'd build two pronto! One for each u-joint. Your probably better off building something VERY strong rather than buying a universal one from summit or elsewhere! Mostly because if these fail on a regular car, theres still lots of hard bits between the driveshaft and your soft bits! Not so on a 7...
Build them as close to the shaft as possible, that way if it fails they are contained and can only continue to spin, and not flail around.
I had the front u-joint EXPLODE on my R1 powered seven at the redline in 6th on a track! Thankfully no passenger, as it ripped through the 3/4" square tubing cage that surrounded it like nothing was there. Then smashed my fire extinguisher, and mangled my passenger side(left side, car is RHD) kirkey seat. The only thing that saved me was the mounts for my driver seat. I made them out of 1" square tube with 1/8" wall. Its what I had laying around in the garage when building them, and wow was that lucky. The 1" tubing was shaped like a banana, where the shaft hit it, and was then deflected to the other side.
On the new 7, the driveshaft loops are 1" round tube with 1/8" wall. Definately not something to take for granted in a 7.
Good luck and enjoy!
Paul