2 years ago I used cardboad in winter only.
This winter I installed a FIA winter grill guard ( fabric with snaps, like a "bra" ) for the top, and for the bottom I put a thin strip of wood laminate down there. ( a scrap I got from something ) In March I got my Insight and I havn't driven the FEH many miles since.
I'm sure I can leave the bottom area covered year-round, since I'm a fairly gentle driver, I have trouble heating up the car in the first place!