Start with specification requirements, what CPU (power consumption, speed), mobo (size, features, weight), memory (size, speed), screen (resolution, toughness,

Start with specification requirements, what CPU (power consumption, speed), mobo (size, features, weight), memory (size, speed), screen (resolution, toughness, size), etc. Once you have a list of constraints (not necessarily specific, but bounded), think about how these will work as shock-proof, water-proof. Will it just be the case that is shock/water proof or will the components be as well. The screen is probably the hardest bit to make rugged since being a tablet it will probably be a reasonable size and therefore easy to crack/scratch/break. So a large amount of focus should go here. Just as a start