I have worked with the Atlas wood reefer, modifying it to make it more correct. It is actually a model of the ACF wood reefer with a fishbelly center sill. (It is the plastic version of he brass car just released by Norm at Protocraft) You have to lower the floor to make it correct - it is recessed to allow the tinplate wheels to operate under it. It is most correct for the C&NW NWX reefer series. It can pass for a MDT reefer if you ignore the center sill or leave it recessed. Depending on your time frame, you have to convert the brake system to AB as it comes with a K brake.
Also worked with the Red Caboose reefer which, as a previous correspondent noted, is based on a PFE prototype. Underbody is closer to what you need, but the car sits high and needs to be modified to get it down to a better level. You have to rework the bolsters on that car.
Nothing is easy in O Scale!
Jim Taverna