Newt was the originator of this idea and I also used this method. Worked very nicely.
It will only work on the stock grip. I had a couple of pics but cannot find them now.
Here's the ACE Hardware numbers:
1 1/2" Slipjoint Washer - 4028338
Sink Strainer Coupling Washer - 4099875
The flat one is used to keep the other washer (grey rubber) on the grip (close to the housing). It's loose enough that it will fall off when it's in the 'off position. The idea is to roll the washer between the grip and the control housing where friction causes it to keep the throttle open. It's enough to keep you from having to hold it open but not enough that it will stick there. It will eventually roll off and you throttling it back is no problem. I sometime forgot it was there and left it in riding around town. However, I am also in the habit of positively closing the throttle and not relying on just letting go to close it.
Where the heck is Newt anyway?