I would look for 10 -12" with variable speed and an adjustable inlet to bring in fresh air

https://www.rimolgreenhouses.com/index.php?option=com_content&view=article&id=2:how-to-size-fans-and-shutters&catid=7:learning-center&Itemid=104

with an intake vent and an exhaust fan you should be able to pull air through an 8x12 just fine. its going to cost a little bit though because you really want a fan and a powered intake vent if at all possible with a thermostat so you don't have to baby sit it all the time and can leave the house from time to time. Those bright sunny days with no cloud cover in spring can really cause heat buildup. And it seems if you have a louver door you can open, as soon as you crank those suckers open, it clouds up, especially if you leave the house, and then the heat kicks on and the windows are open...dont try to run this in summer unless you have a heavy shade cloth to put on it too! its to small to keep cool in summer heat.