I've designed, built and tested a (relatively) compact targeted extruder.
The gearing is 9:31, which doesn't sound much - but the driveway is an bolt, so the gearing is more than you require.
I used an M4x40 cap screw (left over from the mendel build) for the driveway, using a locknut within the drive gear. The gear could easily turn and use an hex bolt instead. The drive is transferred bolt head using three M3 set screws - therefore there should be no problems with torque transfer.
For your hobbing I used an M7x1.0 tap on the shank of the M4 cap screw... but I am sure different choices of tap would function.
The next photo shows the drive bolt's bits. Basically, the order is (from left to right): nyloc, gear, nut, nut, washer, bearing, washer, hobbed gap, washer, bearing, washer, bolt head.
Do not think everything in that photo is correct: I had the M3 screws mounting the bottom the wrong way around, you will need to put both right hand M4 bolts to the lazy block in before you attach the engine and the mounting screws to the carriage should be M4x16. But the drive is correct.
As I am using a stainless steel heat barrier, I used base.stl as a template for drilling holes in a sheet of aluminium, to act as a heat sink. Based on your end layout, you want to tweak the base anyway.
The extruder is contained in the file - so you can take a look at how things fit readily.
Finally, the openscad file can be used to generate a low warp edition of the bearing mount (analyzed), or one using MR84 bearings (untested).
The license doesn't apply to parametric_involute_gear_v5.0. Scad - that document is GPL (http://www.thingiverse.com/thing:3575)