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Shimming the box, and pouring/venting channels.
I've cut some extra pieces of 1/2" foamboard to fill up some of the space around and under the bezel, so as to reduce the amount of rubber I will need to pour.
The addition of the 1/2" foamboard reduces the cubic inches by about 45 CI, reducing the amount of rubber needed by about 2 pounds. And for rubber, a single-pound of rubber can cost you about $15-30, so it all adds up.
The first picture shows the box with the extra pieces of foamboard in place, and the holes where the bezel's pegs sit.
With the bezel in place, the vertical gap between the board and the bezel is about 1/4", which is easy to fill with modeling putty.
The second picture shows how the pouring and venting channels will be laid out.
Normally, for larger (thicker) pieces, you can just have a wide opening at the top, and pour the resin in there, with enough room for the air to escape the mold.
However, for something thin like this, you need to create seperate pouring and venting channels so the resin can get in, and the air can get out.
The second picture shows the pouring channels in Red, and the venting channels in Blue.
Now you may be asking, why are the pouring channels looping down to the bottom of the piece?
That is because if you pour the plastic resin in this fashion, the resin actually pushes the air out of the mold out through the top, resulting in a perfect pour with less air bubbles in the finished piece.
The pouring/venting channels will be sculpted onto the posterboard with modeling clay, and will actually be molded into the rubber when it is poured.
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