Just make sure you have one arm hanging out the window holding the mattress. It's a must when transporting mattresses on the freeway.

In all seriousness don't do it, if it were around the corner then it'd be alright but the wind pressure forces on the mattress at freeway speed are immense and if they don't break your tie down straps they have been known to rip the roof racks right off vehicles.

IMO I would never buy a second hand mattress, the frame absolutely but people do all sorts of disgusting things on mattresses.

see if a local BMW dealer will rent a roof rack to you.

Our dealer rents out all the rooftop accessories.

If you see a faded sign by the side of the road
 

that says '15 miles to the love shack', love shack, yeah, yeah

Short of that scenario...

I don't much like the idea of strapping a mattress to the top of your X5. A strong crosswind gust as you cruise at 70 mph and you could find yourself in a bad dream, tossing, turning, rolling over...

My $0.02 is to call a moving company and see what they would charge to put it in one of their big trucks and deliver it to you.

Subscribing to this thread in anticipation of the inevitable comedy .
A short video to get things moving .
How not to tie down a mattress to the roof of suv - YouTube

And this little ditty
Funky Kings - Mattress On The Roof 12'' Vinyl (1976) - YouTube

sell the x5, get a corolla, use the left over money to buy a new mattress, pillows, covers, frame, box...for the next 10 years? :)

Get a new hitch with a lock, less than $75. Rent a trailer. Uhaul can't charge mileage, trailers don't have odometers...

This will be a disaster for you. There is no way to tie down a mattress that will survive for even a few minutes at anything close to highway speed. You really need to find another answer. Good luck.

amacman, that was funny, and appropriate. I see something like this happening as well. 400 miles is no joke. Even if the mattress is still in plastic, I bet that plastic will go bye bye in about 100 miles. Oh and how long has it been since you guys got a mattress? I bought a tempurpedic 3 years ago, and the plastic has these little holes in it. Like so it doesn't get stale or something. Those hole will make for an excellent parachute.

This all sounds like a horrible idea.

a king size bed is bigger than the width of the x5.