The lenght of the harness is easy to adjust using a pursik knot (a larks nead with another wrap.) It slides easily with no load but will hold once a load is applied.
-Prusik- (and "lark's head.") It's easier to google for a picture and directions without the typos.
What you actually want, though, is a line knot called a "taut line hitch," which is what you tie when putting up a tent, on its guidelines. It serves the needed function of sliding easily when not strained, and locking under load. Sadly, the Eagle Scout in me finally realized that those awkwad teen years might have taught me something useful
The bolt snap itself just slides completely free on the loop of the tow cord, which is what I'm sure DA Aquamaster meant to indicate; his photos show it, but he didn't say it explicitly. The taut line hitch knot is strictly to adjust the tow cord length, not to couple the blot snap to the cord, or to the diver.
There's a really good photo set here (
Dive Xtras - Makers of the X-Scooter (underwater scuba scooter DPV) - info@dive-xtras.com - 425 493 9600 - Tow Cord) of how to rig a tow cord on a technical quality DPV. You might find it helpful. (You might also find yourself with scooter envy, too, but that's a natural occurrence, and nothing to be alarmed about.
)
Bottom line, though - to do it correctly, you must dive with a crotch strap; one-inch widths are okay, but two-inches will be a lot more comfortable and more secure. Crotch straps are very cheap to make -- webbing is available at REI, online
http://www.seattlefabrics.com, or through your LDS, and you just need to sew a little and add the triglide/d-ring...or just buy a kit. If you spend more than $5-7 on the strap/hardware if you go the DIY route, you're doing it wrong.