So what I want to have is some padding in the marked blue area (3rd image), so that my grip would be a bit more firm and wouldn't slip so easily when I have dry palms. In my case my grip holds better with this mouse when I'm sweaty. Also I want to reduce the friction my wrist makes when it rubs against my mouse pad. My sensitvity is 34.6cm/360 and my hand length is 20cm, but my hands aren't really robust. I know this mouse is a bit too small for my hand by the looks of it, but all the other attributes are just about perfect for my needs.
image from the side (with hand). You can see that most of my palm doesn't touch the mouce aka. "claw" grip.
image from below (with hand). I get annoying amounts of friction in the red marked area, almost neglible in the yellow and green areas.
image from the top (without hand). The blue area here is where the said extra padding would be.
If you want it to be really grippy yet not sandpaper lie on your hands I would suggest getting one of two things. You could get any of the 5/10 resole kits http://fiveten.com/products/accessories/resole it's a rock climbing rubber sole for shoes you could cut to size, it's soft and super grippy. Alternatively you could make some form of pad and then paint it with http://fiveten.com/products/accessories-detail/4280-stealthr-paint-kit to add that grip factor that you seem to desire. Bit of a ghetto idea but should work, if you go ahead with it I would love to see pics!
After having abit more of a look this would be the shoe kit I would use, http://fiveten.com/products/accessories-detail/4276-stealthr-c4-kit-half-sole it looks flat but it's super grippy. I use it on shoes for free running/parkour and I was always impressed!
The reason why I don't want to use a fabric is likely the same as every mouse manufacturer's, it doesn't deal well with long sessions and isn't consistent. Simple clear tape resembles closely my mouses build material anyway.
Iw as having issues with my mouse (G9x) as far as being too long for my claw grip as well. I took off the covering things and sanded down the back end to make it as small as possible. If you were so inclined, you could cut the back end off. The circuit boards and whatnot don't go into the back in all of the mice that I have looked at. You could likely cut off about an inch of the back, maybe more. That would cause some issues as well though, but it is an option.