Fun project. I know we were able to do pretty good in our Matrix. My wife probably drove it 60% of the time. Lifetime average was
just under 36 mpg. But, it was manual. My wife also was open to altering her driving style to some degree and normally does beat EPA when she is driving anything.
For the grill block, in winter you might be able to get away with a full grill block. In summer I'd recommend blocking the top and leaving the bottom open.