Our house was built in 1931 and has some concrete steps leading down into the basement with an simple plywood door covering the opening at the top.

The elements and mice come through as they please, so I would like to install an exterior-grade door at the bottom of the steps going into the basement.

The opening doesn't appear to be any kind of standard size. There is treated wood attached to the top and sides, and the rough concrete on the floor. The size (measuring the space between wood frame) is 34 1/2 inches wide and approximately 82" tall (the concrete on the floor is not even).

What size insulated door should I acquire?