c0da, looks good. I have the same car (2006 LX) and have been wondering where I could start with aero mods as most of my driving is on limited-access highways and I could probably see a benefit.
My big concern is making things that don't make my late-model car (with less than 20K miles) look like a home-made P.O.S. I applaud guys with 10 year old cars that aren't afraid to bolt on skirts they cut with a pair of scissors, etc ... but I just can't do that to my new baby Civic.
If you make a piece to block off the bottom, I'd make
several of the same ones and consider some openings in the middle to let
some air through. Maybe make one with small openings totaling 25 square inches ... another 50 square inches, etc ... Making the template (properly) is probably going to be the largest chunk of your time ... multiple copies should be light work.
On most Hondas I've owned, the temp gauge is VERY stable and predictable. Any changes which adversely affect the cooling should be
easily noticeable as long as you monitor the temp gauge.
I'm waiting for some innovative company to come out with an aero kit for these cars ... solid front wheel covers, rear wheel well skirts and a more efficient front splitter that bolts on underneath the front fender cover.
I made a MS Paint image of what I'm thinking of: