Built a fairing for my trike. I find that too wide a track is not a good thing. I prefer being able to get it through my front door and having it narrow enough to be able to overtake on bikepaths. Around 70 cm.
I also kept the rack free a la Go One to allow me to use my pannier bags for shopping etc. Also makes it easier to use a trailer.
I went for a semienclosed canopy. Open in the back and with an opening in the windscreen. This to avoid problems with fogging/rain/snow and thus eliminating the need for wiper. Also provides some much needed ventilation but it still gives good shelter from the wind and rain. The amount of moisture that gets in is still a lot less that what I get from sweating.
Lots of pics of the build, starting here: http://sports.webshots.com/photo/2143407000045660131VVEicS And finished item: http://sports.webshots.com/photo/2451458220045660131MiXujX