Crap, I seem to have lied about having pics to send, and the bike is now in storage at my son's house. So, I won't have any pics of the finished tensioner until I get over there again.
Here is a pic of a leftover scrap of the plastic I used.
I got it at a local plastics wholesaler. I don't how much it would cost, because it was a piece of scrap they had in a barrel marked... "scrap." I was there on other business, and when I asked the price of just the one chunk they said, "Just take it." That was cheap enough for me.
I will give you a rough description of what I did (Please pardon the imprecise measurements. I will have to measure and photo it one of these days.) After a little head scratching over what I expected would be a complicated project, I realized that it would actually be surprisingly simple.
I cut a piece probably about an inch by maybe 1/2" or 3/4" and about a foot or so long (the leftover shown is about 13" so maybe it was that.) Then, I cut a piece about the same cross-section by about an inch or an inch and a half long (I don't remember whether I cut this from the foot-long piece, or if that is what made it a foot long.)
I then cut a trough in each of the pieces the width of the chain. I think the trough on the longer piece ran only a few inches on the rear end and a shorter distance on the front. I just jammed the end of the tensioner in between the bottom run of the chain and the frame (I think at the swingarm pivot.) I pushed it in until the end was at the front sprocket. A frame web near the front sprocket had a hole already in it. I drilled a hole in the tensioner to line up with this hole, and ran a bolt through to hold the tensioner in place. I then used hose clamps to secure the shorter piece of plastic upside down on the rear end of the tensioner. That keeps the tensioner aligned with the chain.
I know all this sounds incredibly flakey, but it isn't quite as ugly as it sounds, and it works very well. The single-bolt mounting secures the unit just fine, and binds it over the frame in such a way that it has just the right amount of upward force on the chain.
I can now be fairly lax in my chain adjustment, and have not had the chain jump off once since installing it.