Jim, yet more examples of your creativity! Good work!
I like your idea of creating the extra friction by threading the line through the bull hitch. I didn't know that one so did a search and came up with
Bull Hitch.
. . . much more robust than the rubber-band idea!
Question: Instead of the anchor hitch (one of my very favorite knots), would a half double fisherman’s (aka double overhand loop) be cleaner? It would have the ends parallel with the vertical lines and thus have a smaller profile.
In fact, depending on the slipperiness of the cord, a half fisherman's (half "single" fisherman's) (aka overhand loop (slip knot)) should do it.
In any case, I suggest not using a stopper for either the anchor hitch or the double overhand (again, lower profile): Both of those knots will bind down tightly and not slip . . . depends on the cord used, though. I would think that the heat-sealed ends should do anything needed.