I recently received a link from Arnaud Vincent to his picasa photo album about the new back bag he created and built to carry F3F sailplane safely to the slope.
The concept is base on a mix of a rigid box and a back bag. The rigid box is made from a transparent PVC panel that is used for greenhouses. It is about 10mm thick, more rigid than the coropplast, and about the same weight.
Then around the box, Arnaud sewed a bag, on measure made of heavy waterproof nylon. Hopefully, he is equiped with a sewing machine, so it helps. The bag received staps at the right place, including some to attach fuselages on the side of the bag.
The box weight is 2.1kg and the bag 900g. This is not light but not that much considering the protection you bring. Here is the photo album Arnaud sent me: