Why is it that when it comes to safety nobody trusts themselves? OF COURSE YOU CAN MAKE YOUR OWN TETHERS!
I read a review recently (practical sailor??) where name brand tethers failed far short of their rated capacity. The ones from ebay... I'd be scared of those. Making a load bearing sling is more complex than stitching in a loop.
I've used tethers made from regular old running rigging 3/8 or so stay-set with bowlines and or figure 8 knots and climbing carabiners. Hey, if it's held a 300sqft main in 30kts of wind it should be fine as a tether on a boat.
If you want something a little more elegant check out Yates. They load test their stuff, and make slings for the military, industry, cavers, climbers and SAR folks. I'm sure you will find something here you can use:
If you don't use yates slings but want to stick with webbing do a little research because webbing doesn't hold some regular knots. The water knot for example is a good webbing knot.
For those of you who don't trust yourselves to make a tether have you ever gone up the mast? Did you use a special personhauler(tm) or did you use your regular halyard with the bowline (or splice) you tied three years ago?
BTW there are lots of quick release/lock mechanism carabiners. Check any climbing shop: Petzl Am'D Triact-Lock Locking Carabiner from REI.com