come on mate i already thought of that!
all the boards are caulked where they join, sides ends the lot. Ive personally sat in it in a strongish wind and felt for draughts, its blooming cosy in there!
my patent ventilation grille isa under that plywood board
the only place where theres a problem is top of the door, im goin to put a length of rubber over that to seal it
It was adapted from a design for 8-10 chucks, down to a 4-6 chuck design