After you shoot, it should get to be automatic to slip an arm through the tubing loop, so you have it no matter what happens. This is easier if you attach the tubing to the front end of the sling stock so that the tubing runs above the notch between your thumb and finger. Adjust the length of the tubing on each side of the butt so that the pull is even. This also gives you more tubing length, ie, more power. This also means you must put the same end of the tubing under you hand, so you shoot consistently. It gets to be automatic.
Wood(Mahogany or oak, something that is hard and won't splinter) is a good material for the stock. If for some bad reason you drop the sling, it floats and you have a better chance of recovering it. Also, after you shoot and have your arm through it, it stays above and behind you, out of the way while you are chasing down the fish. With the shaft in it, the whole thing is negative and sits on the bottom nicely. The butt end is better in metal or strong plastic. A butt end made of wood is a bit larger than works well.
My best sling is oak, has a center hole about double the diameter of the shaft. Its shape is fairly narrow for the back end with a larger head to which the tubing is attached. The back end is easier to hold and the larger front end allows less difference in length between the two sides of the tubing. You don't want the center hole too small or it will disturb your aim. You aim the spear, not the sling. Ktrells pic of a wooden sling looks similar to mine, but mine has more diameter difference between front and back.
