After the glass had cured, I tried pulling the weirs used in it's construction. Sadly the adhesive from the glue gun stuck a little too well to the plug. I guess I got carried away with the glue and used a lot, figuring it'd just peel right off but it didn't!
Ended up tearing some of the skin off the plug in the struggle; so i patched with a bit of smooth packing tape. Next time, I might attempt covering the entire plug in fiberglass beforehand as a way to toughen up the skin.
Anyway, I carried on with the other side, this time just using a few small dollops of glue to hold the weirs onto the plug.
I cut a few depressions with a 3/8" drill bit into the opposing flange there (marked by arrows) so that the two sides of the mold have a means of orienting themselves neatly.