I used a fan from McMaster Carr (# 9191K32)...
Equipment-Cooling Fan Kit with Thermostat, 4.69" Square, 100 CFM Airflow, Steel Guard = $55 plus shipping...
for my Gast 0523 (1/4 hp) air pump. It's 110 volt and has a built in thermostat that kicks the fan on around 80 degrees.
It has been in use for over a year now and works very well. My pump enclosure is just big enough to put the pump in and gets morning to late afternoon sun.
You might consider using a larger fan...McMC#9191K72 ($82). It moves more air for your larger pump. Don't forget to put an air intake in your box so the fan has somewhere to draw the air from.
