Any reasonably study aluminum tube rack should be okay. Panniers and racks use a standard for mounting, so any rack that looks like this should be okay.
An mtn bike specific rack like that one is probably a good choice since it
will be wider to have more clearance, but most bike racks should work if you
use skinnier MTB tires. The important part is something to hook the lower pannier connection on just above the lower mounts. Avoid older racks with hinged parts, these simply won't stand up to the load.
The tricky part is mounting the rack to the seat stays. You can do this using P-clamps from the hardware store.
Here's an example:
If you are using P-clamps for both upper and lower attachements, it would be a good idea to keep the weight as low as possible. You can also get brackets that work with a long skewer on the rear axle, but I'm not sure that's worth it for a single week long trip.
And if you've got money burning a hole in your pocket and want a solid long term solution.