Silas:
Something I have been using quite successfully is hot melt glue. Of course it cannot rub on the bore or YUK! So, it has to be paper patched/paper tube or a wad slug.
I have gone largely to wad slugs even with round balls because I find them easier to load and easier to get good accuracy with from smoothbore.
My solid slug is somewhat undersize currently as it was originally intended for use in thick steel shotcups. I have to "patch" diameter to snug fit into standard shotcups. I also modified the mould so it has a pin to make a screw starter hole in the base of a nose pour TC style slug. After casting I run a short #10 wood screw in then put the slug in a bored CPVC 1/2" pipe coupling then top up with hot melt glue. This makes a nice uniform attached wad. So far they have been shooting fairly well but I will be modifying the mould by boring the nose portion out to about 0.680" with a reduced shank to suit the taper of shotcup petals. There will be some other modifications to eliminate the screw and give more glue surface.
Accuracy is not as good as I want yet but the slugs are tough and the only time I have lost wads is shooting through a substantial tree and if hitting rocks so the slug is basically destroyed. In fact I recovered some yesterday that I had shot when there was lots of snow on the ground and some from our 100 yard sand berm. All could be loaded and shot again.
I have also made forms to add hot melt glue "skirts" to Lee Drive Key slugs and they shot pretty well giving groups about 4" to 5" at 50 yards. Not quite as good as round ball loads but better than as cast Lee slugs so far.
I will post some photos of my recovered slug collection probably tomorrow.
Your idea of melted plastic bottle has been brought up before and just might work. It should be similar to the hot melt glue but harder and able to rub on the bore. Since I have not melted bottles I am not sure how well it would pour but hot melt glue is pretty thick and it works well so it should be okay. While I am using a CPVC plastic coupling with no problems, I think you would need to use a metal form for melted plastic. Bored out aluminum tubing should work. I just push mine out after they are cool and hard. No split mould required.
Longbow