[aerohead]

I don't 'know' anything about these cargo carriers.
As an independent entity,they are aerodynamically 'backwards' as far as 'ideal' drag goes,so you make the leap,that they should be reverse-mounted.
The only caveat that I can think of,is that during wind tunnel testing,with yaw introduced to simulate some crosswind,that they went with the rearward center of pressure of the blunt rear as a concession to straight-line stability.
My opinion is to never use any of them,using a receiver-hitch mounted cargo carrier instead.Preferably boat-tailed.