I was tired of wasting and throwing away so much foam core to make a mold.
I've been using LEGO bricks for my silicone molds lately, and it's AFOL dream! I put a layer of hot glue around the base of the mold to make sure there aren't any leaks! Then spray the inside with a Universal Mold Release so that the silicone rubber doesn't stick to the LEGO brick wall during the removal. I used Smooth-On Silicone Oomoo 30 and let it sit for 6 hours and removed the bricks. You can reuse the bricks for other molds!