Blanking Un-Needed Windows

The buses I am looking at have 3 windows down the side and one at the rear.  These are single glazed windows so the vehicle will be cold in the winter, hot in the summer and condensation is a serious concern.

I am looking at removing the rear to windows on both sides and the back window.  I have 3 options at the moment.

1. Paint the insides of the windows and using kingspan insulation with a thin plywood covering.
2. Remove the windows and replace with plastic (or similar) material cut to the same size as the glass then kingspan and line.
3. Remove the windows and have a sheet of steel welded externally and respray.

The third option is the best one but may require too much work and costs.  I am currently veering towards option 2.  I just need to find a decent material for this purpose.