We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "john royce"
saying something like:

A friends flat is overlooked by other flats and they want to put up some
japanese type opaque paper and also some reflective film ( the type you can
look out of, but people looking in only see a silvery reflection) on to the
window glass.

Could greaseproof cooking paper be used instead of japanese paper (since its
probably a lot cheaper) and if so what would be a way to stick it to the
glass so that it could be easily removed?

Make a lightweight frame to carry the paper and put it in place as
needed.
Window reflective film can be bought from car accessory shops or online.
Shop around as the prices vary widely.