Any such thing as a decent folding MTB?

[highlandsflyer]

You can lay them flat on the roof, after all your box will be a bit higher.

 

[Chris]

Not cheap but you could have S&S couplings fitted to a frame making it into a folder (well collapsible)

Bob Jackson does it and I'm sure there are others.

 

[stewlewis]

MaxxRaxx with 4x4 rear wheel offset if needed. The brackets tilts to allow rear access.

 

[velomaniac]

Dahon matrix, they're tougher than folk perceive especially with the new lockjaw hinge. You can break any folder if you try hard enough but the Dahon is pretty tough.

Alternatively the Montague predecessor the Rudge biFrame in this country, you'd be retro and off road capable. Alternatively get a Airnimal rhino frame and build it up yourself, it might have 20" wheels but is full sus with an 80mm fork

 

[Oval Tech Crank]

Not cheap and not retro but the change bike df602 is a cool folding mtb used to come with xt components.