Cut out the door and reused it.
Framed around the opening with the inner support 1x4's and let them stand proud in the opening 1" for the door to close on.
One thing I wish I had done is trap a "p" shaped rubber bulb door gasket between the 1x4 and the outer wall.
My window was a small self storing aluminum storm window.
Although they are designed to mount outside I mounter it with the flange inside in a heavy bed of 100 % silicone and wood trimmed the outside
