[Christ]

If you can afford some scrap brake line and a tarp, you can make an easy on/off fast back for your car.

You bend up the brake line to follow the rear window form, and then make a second form that follows the deck area, then join the two at the corners to make a triangle shape in 3 dimensions, then buy yourself a tarp and attach it to the brake line frame with some zip ties, installing some fairly strong magnets inside the folds of tarpaulin. The total cost should be less than $30, since you can get brake line from junk cars for free.

You can also make a slightly less strong frame from wire coat hangers, but it will require more reinforcement along the sides.

It's not perfect by any means, but it gets the job done, and looks like hell doing it.