reflextics - well the shiny bubble wrap type stuff

[TWIH]

Some folks are having good luck with using that white foam core foam signboard stuff (just white), or they are using a white cloth over regular cardboard.  White doesn't reflect like the ugly alum foil in windows reflectix does, but white doesn't draw the attention and doesn't soak in the heat that a black felt/fabric would.  If you have ever seen the white vinyl treatments on commercial vehicles, same basic idea, although there's is a perforated vinyl wrap.

I don't think that there is a high temp white spray paint like there is a hi temp black, otherwise I would suggest that.  Best thing for most is professionally applied 5% tint on all windows that it is legal to do so. Or throw a big 15x20 blue tarp over the whole thing, who would notice?