Just read this thread again for the first time in months.
I received this tip during the thread, but the biggest mistake I made in hindsight was not pre drilling the four corners of the bottom before taking my grinder to it and cutting the opening.
And make sure you leave at least a 1/2" lip around the bottom for additional strength. It makes it a little more difficult to remove the guts, but dont just "cut the bottom off". The rubber case is not as strong as you might think. When you cut it open it will need that lip around the bottom to help hold it together.
It will crack easily, so be very careful when applying leverage using the edges.