I actually made my harness. I'm kind of proud of it, actually.
I'll describe it just for the benefit of all, but I hope I can get some pictures soon as they will make this simple process more clear...
I took a ten foot piece of 1 inch strap and put a binder on the end of it, so it has one end attached to this clasp and the other end run through the spring closure on the clasp. Not a good description, I know, but bear with me until I get some pictures.
I put the clasp in my left hand and make a half a turn in the strap so I have a figure 8. I put an arm through each loop of the eight and have a buddy, or I do it if I'm feeling very dextrous, clasp the two ends of the loops, which are now behind me, together with a carabiner attached to a rope. the rope is attached to my truck which I pull. It's so much simpler than it sounds, I swear! All the parts, the strap, the clasp, two carabiners and a ten foot 3/4" manilla rope were about 20 bucks at my hardware store.
If I had to do it over again the only change I would make is to get a wider strap, for comfort's sake. I think the 1 inch strap with it's little clasp is plenty strong, as I pull my 4500 pound pick up with it, but a wider strap wouldnt gouge me so bad.
Also, FYI ebay is an awesome place to buy a thick rope for pulls with, I bought a 50 foot inch and a half manilla rope for less than $2 a foot with shipping.