Zip loc bags defeat desert conditions every time.
I've shot my M in dust storms and not had any issue apart from some dust in the VF. not saying it cannot be a problem with a M, but a CLA can clean out dust if you do have a major influx. Most of the time just shielding the camera a little will ensure the dust just wipes off. If in extremely fine dust, i.e. like powder paint, only fully sealed cameras will keep the dust out.
Holgas are great, but perhaps not the best choice if you want to get reliable results every time in a short space of time.