OK, custom enclosures are going to help alot. Instead of messing
about with fans and non-condensing gases, you could fill the
enclosure with oil. Just immerse the components. Then you have
better cooling, no need for fans or vents (you could use a small
pump but there would be convection currents caused by the
hotspots like the CPU and GPU), plus the oil would also give
some good shock absorption properties. I would make the lower
half of the enclosure a seamless unit with rounded corners. No
sharp edges or welds to spring leaks. Like the original bottom
case without all the holes in it. A rubber seal around the top
would allow you to simply bolt the top half on. Easy access for
maintenance, easy waterproofing. Titanium sounds a good bet,
with rubber around it to help prevent big dents and aid in
waterproofing. Can you work your secret touchscreen solution
into a setup like that? The biggest problem with doing things
this way (as long as you can sort out the screen) is going to be
the battery.