I would do a kamm for sure and plexiglass would work just fine. Depending how far back you bring it if you would either break it into pieces or hinge it at the very top to keep access to the trunk.
The spoiler is probably not hurting anything considering it sits pretty low. The spoilers you have to watch out for are more of the ricer styled ones that increase drag and down force, but down force is not needed at hypermiling speeds
Another thing to consider is smooth wheel covers. I gained about 1mpg consistent tank to tank over having open steel wheels. I can still check fuel pressure and remove the wheels without removing the covers.
http://ecomodder.com/forum/showthrea...tml#post330171