reflextics - well the shiny bubble wrap type stuff

[TrainChaser]

Some people buy that Foamcore stuff (dollar stores often have it) that people use for their yard sale signs (it doesn't flop). Make a paper pattern of the window, cut Foamcore to pattern size, press into window. Once it fits, remove it, glue Reflextix to the Foamcore, trim to size, press into window (held with tension) with the Reflextic facing outward.

Light should not be coming into the van unless you've got some uncovered windows.