First - why would you want to hide the sexy digital readouts? I have the Morningstar 4 Amp and 6A controllers that are waterproof and lack displays.
Letsee - under Prostar 'Important Safety Information' #2 reads as follows:
• Do not allow water to enter the controller.
Under "General Installation Instructions' #1 one reads:
• The ProStar uses stainless steel fasteners, an anodized heat sink, and conformal coating to protect from harsh conditions. However, for acceptable service life extreme temperatures and marine environments should be avoided.
Installation guidelines say:
• Install in an area protected from direct rain and direct sun.
So - if you make a rain & sun guard for it then yes it can be mounted externally, provide adequate cable for drip loops to keep mist & condensation from tracking down wires onto the terminals, and mount in a location to easily keep an eye on terminal board & crimp wire terminators then go for it. But for $150 gadget the Prostar should be somewhere providing "Ooooo" factor