Hello Brian,

I thought about your situation and have a very inexpensive solution.
Get yourself a SO239 chassis connector, 14 gage wire (50') and
soldering gun. In the center solder a 102" length for the vertical
element. On each of the 4 corners, solder a 104" groundplane wire.
Attatch your coax to the SO239 and use a nail to support the top of the
antenna at the peak of the inside of your crawl space. Allow your gound
plane to droop down up to a 45 degree angle. Trim for SWR.
The ground planes will help decouple the RF from getting into the
house.
This is not the perfect solution, but it is cheaper than buying a
commercial antenna. I found this quick reference for a guide...
http://home.att.net/~wizardoz/cbmw/antenna_fabri.html

Hope this helps...
73
Dungeon Master