I finally found a vertical sliding window for the pod on eBay and the seller is right here in Michigan. It's a 14 x 21 vertical slider with screen, smoked tempered glass and black frame. It's priced with a 3/4" interior trim ring, but we need 1 1/2". A quick call to the seller, an extra $11 for the trim ring, and the window was on the way. The trim ring is "mill finish", which means plain aluminum colored, while the rest of the windows as they come from the factory have black trim rings. I guess if it bugs you, you could hit it with some flat black spray paint. I left it as is.
Step 1: remove the door from the hinges and place it in a convenient position on some saw horses. There's 18 screws holding the door on, and they come out really easy.
Step 2: using the interior trim ring for measurements, I made a cardboard template for the cutout. I'll be using a jigsaw to cut the opening, which cuts on the upstroke. To minimize possible chipping to the exterior door skin, the layout was made on the inside.
Step 3: drill a pilot hole for the saw blade somewhere INSIDE the layout line, take a deep breath, and cut away. This is what you'll end up with. There's nothing inside the door but styrofoam so you don't have to worry about cutting through any interior structure.
Step 4: the window comes with a pre-installed rubber gasket, so there's no need to mess around with that gooey butyl tape or nasty caulk. Just put the door in the opening, insert the trim ring from the other side, and drive in the provided self tapping screws. The pod with her new window:
And a view from the inside with glorious light streaming through:
Contact info for this window below. The seller has an eBay store under the name vintek1958. You'll have to call him to get the required trim ring. Total for this window, including the trim ring, Michigan tax, and shipping, was $105.