I had the Frigiking a/c in my 69 280sl. It is awful in every aspect, and I just cannot understand why folks try so hard to retain a US-importer bolt-on (it even uses imperial fixtures). So I took it out and am working on a 'hidden' replacement. Yes, I understand situation is different for a period correct restoration, which I respect.
I sourced a very compact (more compact than the GEN II) evaporator/fan, which I am mounting so it is tucked well up inside the passenger footwell under the wiper motor - it is pretty much completely unseen. I am working on ducting this *only* to the fresh air vents at either end of the dash, feeding the a/c air through via the rectangular openings at the bottom of their duct (you know which ones I mean, see photo below, courtesy of Naj, that I have annotated.) When using a/c you close the fresh air lever (shuts off external air), and cooled air comes out of the vents through the duct I connect to the rectangular opening. No other vents; I think the cabin is so small that thats plenty of a/c to cool it with top on/up (I note a past post about blocking off the rear bernoulli vent on the hardtop).
Issues: 1. Getting ducting discretely over to drivers side; either rigid slim duct/conduit under heater blower, or maybe via hose above, behind the radio opening (deleting the radio) 2. Placement of the rotary fan switch (the thermostat control could in a pinch be tucked away underneath.)