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The basic idea is to use something that will block air from moving through the intake when the exhaust is off without restricting air flow when the exhaust is on. There are pieces of equipment that are purpose built for this that won't be as restrictive when the fan is running. They're cheaper than filters too. With the springs when the fan shuts off the flange closes and when the fan comes on it opens. You can them without springs that just use gravity to close them that are a bit cheaper and even less restrictive.They're about 20 dollars here.
It doesn't sound like you're having an issue with the tent getting too cold though. You'd only need to block the air flow if you had to shut off the fan because your temps are getting to low but that's the opposite of