LJ, Do you recall where you came up with your entrance (bottom) and exhaust (top) areas: 2x22mm DIA, 1x22mm DIA, respectively?
On this design he recommends 8 total 1/2" diameter ventilation holes along the top frame, 1.57 in2. LJ, your exit hole is 0.59 in2. He has two entrances, 4 in x 0.5 in, or 4 in2 total. I do not know if he recommends plugging any of these holes in the winter.