I vote for the PC case fan, move a lot of air, and the larger versions are extremely quiet, and can be picked up in a huge range of sizes.
As far as ugly:
1st PC 250mm Clear Cooling Blue LED Fan 800 RPM 100 CFM 5 Blue LED light-Best Computer Online Store Houston Buy Discount Prices Texas-Directron.com
1st PC 250mm Clear Cooling Fan 800 RPM 100 CFM-Best Computer Online Store Houston Buy Discount Prices Texas-Directron.com
Click on the small images, shows front/rear views. Those are expensive (and large) for a PC fan, but even at the price and size of them you can keep a couple spares at hand.
you can build one or more into a nice little 'plug' that will fit right inside a port, which will only be as ugly as you build it.
For forced air ventilation, I have a pair of 4" fans in a drop board, with some automotive speaker grilles mounted on each side. for looks.
IF you don't mind a little fabrication. The way I did it on my old boat was to drill holes to the bottom of the drop board, route a groove to each edge, run the wires through holes and grooves and terminate them in small spring contacts, then run power through the jamb to small plates. No hook-up or disconnect hassles, just lift the board out which disconnects the wiring, drop them in and the fan is ready to go. Can even be lifted out with the fans running.
The way I built my contacts (cheapskate, and couldn't find a local source) was to cut two strips of copper sheet, curve it a little, then screw them to the end of the board, using one screw to connect the wire. The stationary terminals were just flat strips on the jamb. Originally I had a ball point pen spring behind the curved strip to keep it pushed out, but found I didn't need it.