Proper boxe for surface mounting GFCI outlet for a garage/utility space?
I'd use a 4x4 box with a raised cover. You can get the cover in a dual GFI configuration. You say a "GFI slave", I assume you just mean another receptacle on the load side. If so then you can get a cover with one GFI opening and one duplex opening:



Just so you know, there is a near zero chance that that old AC cable can safely be used as a grounding conductor. Older AC cable does not have the required bonding strip.
See here: How should I connect a grounding wire from a device to this box?

Box: