If your using the openings for light only, I would suggest you just order a custom sized piece of glass to fit in the opening. You can then just nail in some stops and insert the glass. I would order a piece of laminated glass to fit. By using laminated glass you cover some of the issues that you'd need to be concerned about. This glass is safe from breakage if someone falls into it (it's like the windshield of your car) and it will block out a good portion of noise.

If you require the air movement through these windows then go to one of the bigger diy stores and check their stock sized windows. If the windows were older doublehungs then most of the stores should have something close to fitting. This would be the cheapest way even with insulated glass.

Installing these windows would be easy for anyone. You could use your own carpenter on these.

Many thanks for your suggestions.

We do require the windows to open, so the first option is not suitable for our needs. We need completely new windows since now there are not even frames in the openings. Fortunately, the room is not being used now, so we can spend sometime to find the best solution.