IMO a reflector
is needed to have much effect on the sunlight-generated heat. Some sort of shiny foil. We have an external solar screen that helps a bit, but there is still a lot of heat build-up (our dash area is black!). When we stick a large foil windshield sun reflector behind the windshield, there is a marked drop in the temperature up front.
We were able to find a tall folding screen that was big enough for the RV windshield, but you can make one from foil-backed bubble insulation, available at Lowes/Depot and similar, or get an extra-wide piece from RV mail order places like PPL (see Reflective Foil Insulation rolls|bubble core for RVs - PPL Motor Homes
As others have mentioned, a see-through screen or dark mat on the inside stops the sunlight but tends to convert it to heat on its surface, thus heating the inside anyway. However, a reflector on the inside still reflects the light back outside, generating little or no heat in the process.
The sun reflector from CW was no where near tall enough (27") for our big windshield. We eventually found one (I don't recall where - not an RV store) that was 36" tall and that works pretty well. It folds to 7"x36" and we drop it behind the sofa when not in use.