I am attempting to place a frame and door

at the bottom of my cellar stairs leading

outside to insulate from the cold.The opening is 4 ft Wide was thinking of building a hollow frame to fill with insulation to further keep the cold out.

The framing will be pressure treated 2x8

nailed to the concrete walls.door frame would be 2x4 and using exterior plywood

with a 80/20 split. Any suggestion on a better idea.