An Introduction to Box Moulds
Ok, just a few quick words to highlight some of the stuff you're about to see...
The materials I'm using are a piece of MDF board (to pour my mould onto), corflute for the walls of my mould (you can always use some thick cardboard, but corflute is industry standard because of its structural integrity), hot glue to glue the walls in place, as well as gluing the stretchers down (if you don't glue them down, they'll float away and your mould will be uselss), and a box cutter to cut up the walls, as well as clean up the finished mould (the mould needs to be able to sit completely flat, if the mould is slightly skewed or tilted, whatever you cast in the mould will also be skewed or tilted).
HOT TIP! Don't be a big idiot like me and forget to double-check the walls are stuck down properly - you'll see that I get some leakage out the bottom of my walls. Also, the silicone creates a pretty good suction once cured; you'll notice I stick a popsicle stick down the side of my mould when trying to get the mould out. This releases some of the pressure making it easier to get out.