I’m definitely interested in something retractable as well - going to look at the dimensions of a couple. Theoretically it shouldn’t be too difficult to mount one....
I think you may have some trouble finding something that can be easily modified for two reasons:
1) the cargo compartment for the Volt, while nicely squared off and accomodating for its size, is very narrow, as the wheelwells are totally blocked off behind the plastic trim for the EVSE/jack storage and woofer. I can't really think of any small SUVs, hatchbacks, or liftbacks off the top of my head that are as narrow as the Volt.
2) If you want complete coverage so noone can see into the Volt at any angle, you will have to deal with the sharp taper of the area and the curve in the hatch lid as you approach the rear.
Best solution, in my view, may be to take some sort of plywood constructed behind seat mounting system like the one Eagleco made for the Voltshelf, and attach some additional blocks to the underside of it. Then purchase a retractable shade that is the width of the cargo area at the top of the rear seat and trim the shade to fit the taper (using VoltShelf as a template would not be a bad idea). Fold the edges over some sort of thin, rigid material at the back, and attach some dowels that could insert into the cargo net attachment points to hold it steady. I think you'd have to incorporate some kind of mechanism to extend and retract the dowels so you could use the cover in the open and closed positions.