I use #1 pottery plaster. Household plaster doesn’t work for moldmaking!
And the mixing tool is even cheaper than my dollar-store spatulas! (Everything about plaster is cheaper than rubber.)
Here I just poured the plaster for the front gusset piece. The rubber version of the same piece is on the table to the left. My high-tech air bubble removal system is there in the front of the picture.
Once the plaster has set to about the consistency of paste, I scrape the piece flush with the outside of the mold. This is important because I’ll have to start boxing the mold up with Legos again when I pour the large side pieces.
This will now need to sit while the plaster cures.