I grow in a box that's 2Lx2'Wx3'H so it's about the same as a tent.
For a cheap fix for the time being, if you have extra venting duct run some from the bottom of the tent. Make sure the tent is sealed, with the end outside the text run a desk fan in to it. this will help for now.
Think about pressure, If you don't have a inlet fan you have Negative pressure, your exhaust fan is doing all the work and it's also working harder. With a inlet fan your creating a balance of pressure which is giving more air flow around the plants!