A quick modification of http://www.thingiverse.com/thing:40642
The only change at this point is a reduction of the filament guide shaft from the original 3.5 mm to 2.25 mm. This should be small enough to allow to feed 1.75 mm filament down to the hot end.
I sometimes bore out the filament feed shaft and line it with a PTFE tube. The tricky part is holding it very firmly in place so it doesn't pop back up and grind up on the hobbed bolt then send PTFE chunks down to the hot end and clog it up.
Don't ask me how I know about all of this...