Start with the inside mold pieces. Take the bottom piece and lay it on your table, then wrap one of the end pieces and long side pieces against it so the corners meet.
Now use duct tape to tape this all together. As you do this, remove the pins before you tape, but only in the area you are taping.
Cut three pieces of foam that are ½” x ½” and use hot glue to glue them to the bottom, spaced equally apart.
Repeat the same steps as you did in Step 2 for the outer mold. Once you have the outer form taped, you will have the extra step of caulking the inside seams.