Tell us about yourself!
I am not sure why you need playdough. Can't you just make the first half mold using plaster? I guess not but I am not sure why. Also, as joemellin says, the need for a "spill guard" is unclear. Why not just use a tupperware (as you did) and keep the bottom intact, grease oil or soap it and prize the first half out of the still intact tupperware container? The way I had envisioned is1) fill a tupperware half full with plaster2) Press in an object well greased. 3) (Thanks to you) Add some straws for pouring and some small objects around the half submerged object for the "lego-like" fit. 4) Wait till solid. Grease/oil/soap again and pour on another half of plaster. I am not sure of the need for the playdough.