I use an over sized carabiner (with the open end down) and attach the throwable to one of the midships hiking straps. Now that I have finished working on my Hobie 18' magnum, I will probably come up with a way to put it under one of the wings.

BTW, buy the brightest throwable you can find (I always opt for the closed cell foam ones) and make sure that you put at least 6-8" of SOLAS reflective tape on it. With the bright color, you can find the throwable easily if it falls off accidentally. Also with the SOLAS tape, the person in the water can hold the throwable up in the air and then the person in the water can be found more easily....even in significant wave troughs in the dark.

Last thing, although is is unlikely: I always stencil my boat name and origin on the throwable, in case I loose it or just leave it on the dock.