I tried to look for building materials second-hand. And I found an ad for these mini windows. I think they were showroom examples before, because they had handles attached to the top. This was convenient as I went to pick them up using public transport and I had to haul them back on the bus and train!
Here's my version of window installation: 1) get it level in the opening using shims. Peel back the rubber joint and screw through the wood frame into the structure.