Zebra Pen, M-701 Mechanical Pencil, 0.7mm Point, Durable Stainless Steel Barrel, Knurled Metal Grip, Refillable, STEEL Series, Ideal for Tactical Use, 1-Pack, (59411)

Nice setup! For your pencil, might I recommend the Zebra M-701 mechanical pencil? It has all metal construction, so should fare better being sat on and generally abused but more importantly it's only about $10-$12 depending on where you get it. Even if it doesn't survive your back pocket environment, you haven't broken the bank.