If you'd like to tie your own bungee necklace, the Water Knot works quite well.
Take a length of bungee, tie two loose overhand knots (A & B) separated by 3-4" on one end, then run the other end through the first overhand knot (A) and tie a loose overhand knot (C) just inside the first (A). Then run the end through the next knot (B) and tie another overhand (D). Tighten as necessary (A matches up with C, B with D), leaving a small loop between AC and BD for the mouthpiece of your reg.
Of course, you could always order an assembled one or use zip-ties to make your own.
I chose the first option, but after one of them untied, I decided to learn how to recreate it.