For a box with a response down to 80-120 Hz or so you don't need to tune it; a simple sealed box configuration is best. A transmission line is also viable but I think design wise that's quite a bit beyond your present skill level; if you can wait a few months my Transmission Line Array will be appearing in audioXpress magazine, and after that on my website.Your receiver will handle the crosover duty but you need a separate amp to drive your sub.
Your tweeter array needs to be close to the midwoofer array hieght so that they will have the same response characteristic at the crossover frequency. You will need a crossover between the two lines with a minimum 18dB slope.