Years ago we converted a van we had into a camper. To cover the cab's front and side windows I installed a curtain roller track in the ceiling running all the way along/above the dash and along/above the cab's side windows.
This track had rollers with metal loops. My wife made a split curtain for this track and sewed it along the top to each roller loop. Each curtain half went from the door post on each side to the middle of the dash - joining the other half for a complete curtain covering for all three cab window areas. Each half of the curtain could be slid back to each (driver and passenger) door post for storage when not needed. This was a real convenient setup for blocking the cab windows without having to store someplace and install/uninstall cardboard window covers.
Now - in our Ford van based Class C RV - when camped I quickly install one of those heavy plastic white outside covers that perfectly covers both the windshield and the side windows. This is the best way to do it because it keeps the cab at a more even temperature since outside cold or hot temperatures cannot heat the glass for transfer into the RV's interior.