The one Camptown sells now under Marjean's ownership is very nice with a padded top piece and snaps like the one Whitney shows so you can leave it on the cart and don't have to wrap and unwrap it every time you hitch. I have one by Iowa Valley Carriage that I'm very happy with as it's a bit less obtrusive and although beta actually blended into my leather show harness quite nicely. Few people noticed it was there when I used it for a season to break Kody's bucking habit.
And yes, you can make your own temporarily by using a leadline. I've seen that up here pretty often. You just tie one end around the shaft, bring it up over the rump and wrap it once around the backstrap between the breeching hangers so it can't slide forward or back, then down to the other shaft and tie it off. Viola! ("Vwa-la", not the instrument. *LOL*)
As far as adjustment, you want the strap loose enough that the horse can trot and canter without restriction but if he tries to raise his haunches in a real kick or buck he'll hit it. Generally that seems to mean enough slack that the strap is drooping slightly between the shaft wrap and the haunch and I can raise the backstrap off the horse a good three fingers or so. It should be similar to the breeching adjustment.