Well, here's my suggestion for a home-made window:



The 'window' is just a sheet of plastic (argue amongst yourselves whether polycarb or acrylic is better) which is hinged at the top with a piano hinge and seals onto the outside of the sidewall with a weather-strip all round. It'll need a cap over the top to stop the top seal from getting flooded.
A screen fits in the rectangular opening, but isn't shown - it's hard enough showing the detail without trying to find some way of modelling screen material!
The stays/catches are also home-made from 3/4x1/4 and 3/4x1/8 aluminium strip. The stays pass through two holes in the sidewall that have keeper strips mounted at their bottom. When the window is open, a notch in the stay locks into this keeper strip, and when the window is closed, the two stays hinge down to press against the keeper strips and hold the window tight against the seal. Clip the bottom of the stay to the wall (not shown) and you've got a secure lock.
One possible problem is where the stays hinge on the brackets that are fixed to the window - ideally these need to be rivets, so that they will pass through the small holes in the sidewall. If a bolt was used, the hole in the sidewall would have to be bigger and maybe big enough for bugs to come through.