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