The two halves of the mold weren't sticking together as tightly as I would have liked, so I took some pieces of plywood and some rubber bands and strapped some reinforcement across the front.
The horizontal pieces of plywood are loaded across each end with some rubber bands tied together, which go around the back of the box.
The vertical piece of plywood is the only piece touching the posterboard underneath, and is the piece that's actually applying pressure to the box.
If I had more pieces of plywood, I would have used three pieces vertically to spread the load better.
This ensures that the two halves of the rubber mold press against each other with enough force to make a good seal.
Next, I'll measure out the resin, mix it up, and make the pour.