Box build .... have cpl questions

Building a box 4 feet wide 6 feet tall and dont know if i should go 18 inches thick or a little thicker like 21 inches or 24 inches... this is for moms on bottom and babies on top... my other question is im going to make it pretty air tight but i have no clue what kind of fans to use for ventilation and where i should put them... im building a 4 feet wide by 5 feet tall by 2 feet thick box right next to veg box for flowering... and i need to know where i should install the fans at in it... id like to hook it all up to a carbon filter with a vortex for smell... can someone who has built box setups please help out and inform me on what i have asked.... thanks