First off, Welcome to this wonderful hobby!! You've done the hardest part, by getting jars to be 100% colonized... Congrats!!!... now, just a few more steps away from enjoying the fruits of your success!!
Cakes don't need THAT much Fresh Air Exchange, IME. More imporantly, is to make sure that the temperature and humidity are correct. Humidity should be 95% plus, and temps should be between 70 and 80 degrees for fruiting. If you just open up the lid once a day, or once every two days, that should be plenty of air exchange... no real need to 'fan' it. If you're worried about it, you could grab some Polyfill from the craft store, cut a 1/4" hole in your chamber, and fill it snugly with polyfill. Or, if you have a fish tank pump handy and a couple feet of tubing, you could just run that into your terrarium and set on a timer for fresh air exchange. It's not necessary though, unless you're going to be away for over a few days and/or want an automated setup. There are many types of designs on this forum, but the simplest, cheapest setup is simple damp perlite in a plastic tub.
Good Luck with your hobby!!!