Sports auto shops sell foam sections which are used for clipping onto roll cage bars as used by rally / race drivers in their cars. These come in varying lengths, but are aprox 80mm diameter, and will adopt the lateral curve of the rear window when placed and the roof lowered. They are by design extremely light-weight (race drivers have an aversion to weight) and will easily store in the boot or in the roof well behind the seats. I have been using one for the past 9 months, and can vouch for their "non scratch" properties on the plastic window. It is impervious to water, and can be wiped down with a damp cloth or tossed into a bath of water for cleaning when needed. And for the colour coded types...it is available in black too...
