A mold box can be made with a wide variety of materials. It is simply a containment area for pouring block molds. It can be a Tupperware container, a bucket, plastic bin, or any other non-porous container strong enough to hold the rubber. Many of these standard containers may not be the right size, and you don't want to waste rubber filling a space or make the mold too thin.
You should allow at least 3/4" around the perimeter of the part, and pour at least 1/2" over the part. If the rubber gets too thick it becomes harder to flex and more difficult to remove the cast part. Sometimes it may be hard to get the mold out of the container - you may have to cut the container off.
For these reasons and others, most of the time you will need to make a custom size or shape mold box. One of the best materials is a 3/4" thick Melamine laminated particle board (a white laminate board found at most Home Depot, Lowes, or Menards.) The lamination makes the board smooth and waterproof - a perfect surface for rubber. The one problem with melamine particle board is that it does not take screws well - a lot of breaking and splitting. A quick fix for this would be to use the melamine for the bottom of the box and then use PVC vinyl molding for the sides. It takes screws great, never splits , and is smooth and waterproof. It comes in a variety of widths from 1-1/2" to 7-1/2", and 8' lengths. It is expensive but can be worth the extra cost - no splitting or breaking, and if you slice it thinner to 1/4" or so, you can easily flex it to make curved shaped sides for your box. We recommend using #8 drywall screws, 1-5/8" long for most boxes. You should plan on taking the sides off the box to remove the mold, which is why we rarely nail a box together. Using a pneumatic or electric nailer is tempting because they are so fast and easy to use, but once the box is assembled it doesn't come apart easily, and you can end up destroying your mold box.