I use the Mutt Muffs on my Fox Terrier. I followed someone's suggestion of starting the engine before putting the Mutt Muffs on the dog so he can immediately know why he's getting these strange things on his head.
I thought at first they were a little overpriced for what we received, but hey, if I were to try to make something, it wouldn't be as nice as the Mutt Muffs. YMMV
Longest trip so far has been 1hr 15 min, and he fell asleep, so they must dampen the sound somewhat.
I have no experience whatsoever with a King Charles Cavalier Spaniel.
