Hrm, can't say I have particular experience with custom-made ones, but there's something to be said for the 're-used plastic bag' concept: just poke hole, rubber band to lens hood, stretch another hole over eyepiece: you can carry a few of them if need be.
For a big trip like Yellowstone, something special-made would certainly not be out of the question, of course. But I'd bring a plastic shopping bag or three, anyway. To have a place to carry your wet rain cover, (not to mention anything else that's likely to get wet) if nothing else.