Gustav - There are other options besides the top shaft for the Maxx. There is this HPI shaft which is what I use for my buggy's slipper clutch:
http://www2.towerhobbies.com/cgi-bin...4&I=LXFKF8&P=K
It is 6mm, so it is stronger than 5mm (Maxx top shaft). Also, most outdrives fit on a 6mm shaft, so this would fit without extra parts. The shaft would need to be cut down to fit though, and the sides would need a flat spot put on.
David used to use the Dominator slipper clutch. He said it didn't really slip at all. My reason for making one was that it is cheaper to buy the parts and put them together myself. Also, the spur gear is an inexpensive MGT gear, and there are a number of sizes to choose from (46t, 49t, 52t). It is a direct fit.
The idea of a slipper-diff has me thinking. I might be able to figure something out, but only time will tell.
EDIT:
Just thought of an idea to get a slipper on, but it will need the details worked in. Basically, the slipper plat/pad(s) mount onto the diff cup. Then the gear part would be tightened onto this "unit" (somehow, that's one of the details). The whole slipper-diff would require a larger space than a normal diff, but that can be figured out later.