Custom front and back for modular paint stack

This allows for the creation of custom fronts and/or backs that work with the Modular Paint Stack (MPS). Since these can get rather tall and skinny, a brim is generally a good idea. Smaller sizes can print fine without a brim but larger sizes risk tipping over. See https://github.com/IanGClifton/modular-paint-stack for more information about the Modular Paint Stack system.