I would recommend that Thule Gauntlet case (I have one and have been using it for almost 2 years) and some good panniers, don't buy the bargain bin stuff, panniers shouldn't be flying off. Ortlieb or Arkel would be my choices for quality panniers that are actually low cost (as they last forever and are well supported so long term cost is super low but initial cost is a touch higher). The E-Mate from Ortlieb fits my 13" Thule case quite nicely with space to spare and my GT-54s from Arkel also hold it fine.

If you didn't want to go the pannier route and weren't dealing with weather I would just carefully strap it to the rack with my Highland Fat Straps (the best bungee cords out there) but I would much rather have it in a good waterproof pannier (just in case and I do generally turn it off but I would say probably not needed as it is an SSD but it turns on quickly enough I would rather just have it off.