Originally posted by mp3oplecarrier
This box has extra room around the sides so I can leave room for the pouring and venting channels, but normally it's reasonable to have an extra 1/2" to 1" of room around the sides.
In this particular case, I have left plenty of space above the part because I am expecting to rebuild and extend the duplicated part by about 2 inches in height.
The finished extended part will be placed back in the original mold, and the sections of rubber that lay under the extended sections of the part will be cut out and new rubber will be poured into the cut-out sections.
In this way, the original mold can be modified and extended, without having to re-pour an entirely new mold.
And to a certain extent, the box itself lends some strength to the finished mold, though in most cases the box is cut apart to remove the mold.