I have managed to make one similar to the one from boxer design,took about an hour,the top cover has to be slightly modified to provide clearance for the bigger twist grip but its easy enough and fully reverseable if not satisfied,
costs nothing to do as well.
basically its a bit of lorry mudguard plastic the same width as the throttle grip with a cable groove cut into it and fixed to the grip with 3 very small self tappers.
the top cover needs to be gound away on the inside where the screw fixs into it to provide for the increased circumferance of the throttle grip.