I started working on one for my XR7 before getting sidetracked with the engine/trans build. Honestly, I probably won't get back to making the loop until this winter, but I will definitely post up pics and info somewhere on these forums when I do.
what is the requirement? could you heat up some re-bar and bend it into shape?
....must have a retainer loop 360 degrees of enclosure, 1/4-inch minimum thickness and 2 inches wide, or 7/8-inch x .065-inch welded steel tubing, securely mounted and located within 6 inches of the front universal joint for support of the driveshaft in event of U-joint failure.....
That's straight from the NHRA rulebook. Making one out of re-bar will get you sent home.
If you're running on street tires, in a 14 second or slower car, most tracks aren't going to check for things like a driveshaft safety loop. However, they do tend to be pretty picky about the quicker cars with slicks.