I thought I had the perfect answer; a piece of old clincher. But as previous posters implied, the tube wore against the edge of the boot until it too flatted. 3M plastic tape over the boot didn't work; it eventually cracked and the new tube flatted in the same spot. And the dollar bill I put over that today ended up being ripped in half (good thing it wasn't my hundred).

I'm currently trying a patch of vinyl fabric; which is fabric reinforced, but soft with a slight stretch. I'll let you know how it goes. Rather than superglue, I'm using Shoo Goo as the cement. Works great with no prep, remains flexible.

I have to say I'd be skeptical of putting a plastic bottle patch in there, knowing now how sensitive these road tubes are to sharp edges.