Your chassis broke because you didn't brace it correctly. don't worry about bracing the bottom... just the sides. If you look at the vertical pieces of the chassis in your picture, you can see the bracing only covers like 1cm on both sides of the cut line. Make a long skinny piece that goes lengthwise along the vertical piece of chassis, so you have like an inch on both sides of the cut line. The bracing you did there wont cut it.