I have used a piece of foam insulation hanging by duct tape over inlet opening as an inlet. when fan kicks on it will open inlet. inlet size needs to match CFM capacity of fan. if I was doing this I would make 2 inlets a have 1 sealed down in colder temps and fan at lower setting and have both available for warmer temps.

That's some good advice. Earlier today I looked at some nice vents at The Home Depot (both of which sold for less than $20) which will work just fine. Even though the appearance of the grill would suggest less air flow, as long as the openings are there and your exhaust fan is powerful enough to pull the cooler air in through your intake vents (inlets as you say), the airspace will lose temperature!

The exhausted air has to be replaced by incoming air. inlets need to match CFM of fan