Chris and everyone else the grill block would be for the upper two slots and the lower slots by the fog lights. Done from outside the car, no need to open the hood.

Obtain some 1 inch pre-formed foam pipe insulation from Home Depot/Lowes, about four 3 foot long pieces of the cheap grey type. The insulation is pre slit, just cut in in half length wise so you have two pieces from the one. Fold a piece in half and insert into one of the grill openings. Make sure you go from end to end, the tighter you block it the better. For the lower grill the very top and bottom openings might not take the folded insulation, just cut in half again and insert.

The grill block serves a couple of purposes. It prevents air from flowing into the engine compartment and cooling the engine. When you park it helps to retain heat for the engine. It will also help on the real cold days provide some extra heat for the passenger cabin.

If the temperatures are going to be above 55 degree remove some of the lower grill block, that will help cool the inverter. Don't worry about overheating. In stop and go your engine is on and off, if it gets to 176 degrees, where the thermostat starts to open, you are lucky or trying to cook your passengers with a high heat demand. On the highway, after a couple of miles, use as much heat as you want, that will probably keep the engine at or below the full open thermostat of 190 degrees. And if you do start to overheat (never heard of a Prius overheat) the fans come on at 212 degrees and can draw air from the gaps at the sides and bottom of the panels.

Wayne