You don't have to drill through the sides of the cab from the outside.
I joined my LCD monitor to a horizontal piece of wood via VESA mount. The width of the wood matches the width of the inside of the cab. Then, similar to fenring's photo, I glued and screwed smaller 'blocks' of wood to the inside of the cab, from the inside. After that, I could mount the monitor/wood to those blocks by screwing wood to wood.
You just have to be sure on your measurements so that your monitor doesn't end up too close or too deep.