07 Prius really bad mpg

Yes, but don't just leap in with that. I would only start blocking when temps are staying below 50F, and unless you're able to monitor coolant temps (which isn't possible with just the stock Prius gauges) I would keep it to 50% block max. Also, determine where the inverter's radiator is (I believe it's at the bottom on second gen??), and avoid blocking that area.

Foam tubing, the type used for insulating plumbing pipes, is commonly used. I've found velcro tape a good method for securing the tubing, easier to add/delete tubing, compared to zip ties.