What can I get for panniers to commute with my 17" laptop?
Having a 17" laptop does limit your options.

If I understand it correctly, you want to carry your laptop on your bike (not on your body), in a bag that gives it some impact protection.

If you feel handy and are willing to throw some money at the problem, I'd recommend getting a hard-shell case filled with egg-crate foam, drilling holes in the case to install that Arkel mounting kit (or a different mounting kit), and sealing the holes with silicone caulk. I did a little poking around and found a well-known hard-shell case brand that does have briefcases for 17" laptops.