I use Trident brand Neoprene Cement, but any one that comes in a tin with a brush cap will do. You can also use super glue or gorilla glue, as long as the glue stays dry when it gets wet again.
As for the length of bungee you'll just have to eye ball it. Take a length and make sure it wraps around the post of your mouthpiece at least 1.25 times. That should give you plenty to play with. Make sure it's snug before your tighten your knot. When you tighten your knot, you'll get a little bit more bungee in your loop. So if it's loose before you tighten, it'll be too loose when you finally do tighten it. You'll have a bit of extra hanging off your knot which you'll need to cut and singe with a flame. It's hard to tie bungee unless you have a bit of extra length.
After you tighten your knot, loop your bungee around a D-ring then slip your mouthpiece through to test it out.
Remember bungee can stretch 100% it's original length.
