I've been doing something similar for several years now using Flipbac stick-on grips. For the front of my Leica, I use the Flipbac G4 and for the back I use one half of a Flipbac G3. Much cheaper and less bulky and cumbersome than a Leica grip or any of those Thumbs Up thingies, plus my hotshoe is usable.
Couple of things to keep in mind--I use 3M double sided camera tape (the stuff used to affix rubber panels to camera bodies and sourced on the 'Bay) to attach the grip. Make sure to clean the area really well with rubbing alcohol before affixing the grip/tape.
The G4 has a small void on the back that may cause the grip to slip and move. To reduce/eliminate this, I filled the void with a small rolled up wad of double sided tape. When you cut the G3 grip, you are left with both a curved sided and straight sided thumb rest thingie. Pick one or try both to see which one fits your thumb better. 3M tape should not damage your camera's finish.
I've done this on a couple of M8's, an M9 and a couple of M 240's. No need to add the rear bump on the 240 as it already has one. Grip gives me just enough leverage and fill under my hand without adding bulk. If it moves or slips, just peel off, clean area with alcohol, and reapply new tape. In a couple of years of use, I've lost just one rear rubber grip, but the other one has stayed pat.
