Airline crate. VariKennel, not the knock offs.
I've seen dogs that just put their paws on the side of a fabric crate and it flips right over. Not to mention just two bites will open a hole in a fabric crate. And offers no protection what so ever if in an accident.
Airline crate is chew proof and if used in a vehicle will keep them from harm in an accident. And X2 what BCSnob said about a crate that contains any bodily fluids a dog can offer up.
My friend was in a horrific accident coming back from a dog show. the TT jack knifed and the tow vehicle flipped over. The dogs in airline crates survived the ejection. The dogs in the wire crates did not. They exploded upon impact and broke apart and sadly some of them the broken wires of the crate themselves became lethal weapons to the dogs.
I've since switched to all airline crates. I have a set for each dog in the MH and another set in the Toad. They are always crated in vehicles in motion.
For those concerned about space when not in use. The airline crate can be broken down and nestled and used as an extra storage tub for RV stuff.