I'm building a D-Bot printer; This one: http://www.thingiverse.com/thing:1001065. I'm using V-Slot extrusions from OpenBuilds for the frame.
When it came time to tap the extrusions for M5 screws, I needed something to help me keep my non-self-guiding taps perpendicular. This was my solution and it worked very well: a guide for an M5 tap. It slips onto the end of the V-Slot extrusion and you put in your tap and start. I generally let it get going, then pull the tap, remove the guide, and finish without it - you need to back out a bit every couple of turns to clear the waste and keep the tap from jamming. Once you get the tap started, you don't need the guide anymore.
I used the profile http://www.thingiverse.com/thing:884966 created by SanderSae for the extrusion shape. I re-worked some of the values so I could difference() the shapes to get the correct profile for the slot inserts. Thank you SanderSae.
I printed at .30, but there's no real detail that requires anything finer
Build it in any material you want. It doesn't have to be very strong because it's just a guide and you shouldn't be putting any pressure on it when you're using it.