Spray the box and original with mold release. Pour parts A and B into a disposable container and mix thoroughly, 3-5 minutes, scraping the sides. Try to avoid mixing in bubbles. Put the mixing container in a bell jar, and evacuate the air with a vacuum pump. Pour it into the box, with the original, slowly and smoothly from a foot or two above the box to minimize bubbles.
Tap bubbles to the surface after pouring.
Let mold cure for 13 hrs with good ventilation. The rubber mold can be cut with a sharp knife if absolutely necessary. Let cure for 15-20 minutes, remove part. Repeat plastic molding process as necessary. Parts can be colored by mixing pigment in with the plastic resin.