This is probably one of the most useful, yet simple designs I did for the ultimaker, in addition to the XY "bananas blocks". You can read about the list of improvements on http://www.tridimake.com/2014/01/features-and-improvements-for-a-homemade-ultimaker.html
This one makes life easier for sure. Fine-tuning the level of the Z end switch is a chronic issue, especially as you switch among different nozzles or bed thicknesses.
In this design, the carriage slides tightly in two concave rails of the outer frame, which is secured on the case by four outer screws. There is NO way the carriage can move otherwise than vertically, as it should. Neither the carriage nor the micro-switch do touch the plywood for a fluid movement.
Most of what I have tried so far proved to be unreliable in this regard, probably because of something that (tried to) slide against the internal plywood wall. The only parts here that are in contact with the printer walls are the fixed parts.
I designed it for an Ultimaker, but it easily adapts to others. The openscad source code is included as usual, in case a change is needed (here is a very easy tutorial if you need it: http://www.tridimake.com/2014/09/how-to-use-openscad-tricks-and-tips-to.html)
The main screw
I used a long M4 screw (actually a threaded rod), about 60mm long. Insert it through the hole of the top horizontal bar of the frame, then screw nuts on each side, by leaving just enough room so the whole screw + nuts rotate as a whole without any vertical play.
These nuts can be glued to the screw (as I did, after willingly damaging the thread a bit with a pair of cutting pliers), or you may use two pairs of nut and counter nut instead on each side of the top bar (as the openscan screenshot, a bulkier solution). Both ways are better that using nylocks, which would eventually slip and start to move on the thread in the long term.
Note that you could use an M3 screw for less travel per turn (in which case you'll have to change the value in the openscad source code). In my opinion, M4 is a good trade off when you change your printing bed often, because it's faster to tune with an M4.
I used my thumbnut design for the screw head, which can be customized at http://www.thingiverse.com/thing:70494 I chose small diameter and wings so that I can turn it quickly between my thumb and index fingers.
How to mount the rail frame
The nut holder (http://www.thingiverse.com/thing:44118) was quite useful to help fixing the Z adjuster rail on the printer frame.
Just make sure how you mount the adjuster. The head should protrude from the top of your frame: either use a sufficiently long M4, or use a threaded rod cut at the appropriate length (as I did here in the actual device). Mine goes up a bit too much I guess. You could also use a regular screw with existing head, then use a screwdriver to tune the Z. In this case you do not even need the screw head to protrude from the top of your printer.
The carrier hole is tight, so you can screw the threaded rod directly by force (use quick forth/back movement to tap the hole when needed), but you can glue screws around or heat-sink them in the carrier if you really need it. Just make sure that the carrier slides as a nut would on the threaded rod when you rotate the latter.
The micro-switch is screwed to the carrier from the other side and though the wall of the printer. Just use screws long enough for the thickness of your printer wall. The two slabs on the carriers are there to prevent squeezing the microswitch and carrier too much against the walls, but do not tighten the screws too much (the microswitch must be allowed to slide vertically without much effort, being held by the carrier on the other side of the printer).
Finally, you can prevent lateral freeplay on the carrier thanks to the outer oblong holes on the frame. The best way to proceed is to slide the carrier at the same level as the fixing frame holes, so you know how much pinching you need before yhe screws are tightened on the printer frame. Then, move the carrier to the height of the other pair of frame screws and secure them again. Actually, no piching was required in my case, as the design is already tight (and in any case, horizontal movements have no impact on the end result, only vertical sloppiness would have).
In the end, it will only be 4 more holes in your printer frame. I did not measure the screw length, but they are quite common and you'll sort it out easily.
Hope you like it and that it makes your hobbyist life easier :)