I wouldn't bother. Plaster frightens me, especially if its green.
I do lead casting - bullets, sinkers, jigs, etc... you can't or shouldn't use plaster for the reasons mentioned above for lead and that's only going up to 800 degrees max. I've made moulds using a product called 'water putty' which dries similar to plaster, and has to be heated to 400 degrees for a few hours to drive the water out. It worked for casting lead, but the moulds themselves burn up fairly quickly and would absolutely not be suitable for a forge. Like I said, don't bother.
My first gas forge I made was with a paint can, lined with 3" of refractory wool coated with ITC 100, and powered by a bernzomatic torch that I adapted to a grill tank. It gets HOT, but only heats about 3" of steel at a time.Materials don't cost me much, we have a refractory supplier in town and the salesman was able to sell me scraps and leftovers for cheap. Fun experiment, but if I were to do it all over, I'd build something bigger with the Reil-style burners (which I'm currently working on).