Hi,
I have used the liquid glue/melted plastic "goo" a number of times. I have found that the plastic leftovers from vac-form kits work well for me. I wait until a Testors cement bottle (the kind with the brush in the lid) is about half full, then add the vac-form bits over a period of time, mixing well, until I get a goo that VERY slowly oozes off the brush when held vertically. The mixture saves indefinitely (years) in the Testors bottle. I've used it to fill gaps and also for purposes like building up the non-turret nose of a couple of post-war PBY's. (Cansos to me)
I've also had reasonable success using the mixture in rubbertex molds to make small parts. You just have to make sure you poke it well into all the crevices of the mold. "Painting" the mold surface with liquid glue also helps this process. I've even used this to make corrugated wing panels for a Junkers 52 when converting from tri-motor to single engine. Had to fill the holes in the wings where the engines were.
Hope this is of some use.
Mike