There are a couple companies that make these. I used them for quite a few years. You can leave them on and just rotate them out of the way if you don't feel like using them. One was called a "Throttle Rocker" and the other is called "Cramp Buster". The Throttle Rocker is alittle wider and shorter then the Cramp Buster. They work good for their price............
I've never used them before, but it looks to be pretty useful for longer rides on the highway and open roads when you're constantly on the throttle for a long time. The wrist gets a bit tired after a while.
I've got a Throttle Rocker around here somewhere. I accidentally knocked it off my DL1000's grip while slowing for an exit once.
Rode about 4 miles exit to exit to get back to the point it happened. Luckily, it was still in the emergency lane unharmed. =)
On a long trip and in a pinch, you can make a similar device out of a heavy rubber band and some popsicle sticks.
Looks weird, but works and you get to eat a popsicle or two!
So it really is that simple to fab one up it seems.. Although they are really cheap to buy, I would actually be interested in making something instead to get on there.. add a touch of personality while you're at it right?
Aside from Throttle Rockers, there are Throttle Locks too; basically something to keep the throttle fixed at a particular position. There are plenty of them out there, but some people like to use a huge O-ring.