I use a muzzle bungee:
Spearfishing and Freediving Equipment - Lines & Fish Stringers - Muzzle Bungee - Technosport Inc. And then either mono line or dyneema line from that to the spear.
As for the bands, a pic of your gun would help to see how it is rigged now and to see the gun in general. Most likely you could buy rubber and tie your own wishbone in and make them any length. You might even be able to change it to say a single 20mm band?? Not sure of your spear diameter and everything.
As for the loading, I have a 90cm gun. I have to reach out with one hand and grab the wishbone and then put the butt on my chest and then grab with the other hand and pull it down. You'll get it I'm sure. Just be careful with the gun, I wouldn't even set the wishbone into the notch when practicing to load it, and definately don't get it over the notch and just snap it, or let it go quickly. If the trigger got bumped or the spear not locked in, you could fire the spear while loading. Make sure it's secure.
Good luck! Oh and welcome!
