I have a bed frame (60" x 54") that won’t fit into my car. I have to transport it on the roof of my Buick Century. We are driving 12 hours with this thing on the car! What is the best way to secure it?
If you can’t disassemble it…I would recommend you get canoe pads to place on the rails, maybe two sets to be safe, and tie it on like a canoe. Google canoe tie down tips for help if this doesn’t make sense. I don’t see a wind resistance problem, but drive slower and check frequently. If you think you might need a roof rack in the future, now would be a good time to invest in one. You can extend rack width with 2x4 as needed more safely.
example http://www.merrimackcanoes.com/canoe-tips-transporting.html
Personally, I’d just buy a new one when I got there…at less than $50 it’s not worth the hassle for that distance.
In addition to Dag’s excellent idea, I’d like to suggest that you be sure to use good bungie cords or nylon tie straps…in abundance…and strap them down tight. 12 hours on the highway can cause rope to come loose, especially woven nylon rope.
I don’t like the bungee idea, still too flexible under the air flow pressure encountered when driving.
Stick to a ratchet style tie down or rope.
When laying the frame on those canoe cross bars, tie it tight ( rope ) to the to the bars where it crosses them then use ratchets or rope at the corners as well.
You’ve likely seen many “funniest home videos” etc showing people toting mattresses and plywood and such…
don’t do that ! 
I might add, just to get technical, I would as you say, avoid stretch by using a polyester rope not nylon for low stretch one time ties and low stretch nylon straps to wrap under door jams w/o roof rack. Being a sailor, I’m a knot fan, even with straps but would recommend if you’re not to us ken’s suggested ratchets. Remember that if you have to tie knots and aren’t sure how, tie a lot of them, and leave nothing dangling. I like the “trucker’s hitch” which keeps knots well out of the way of mechanics and problems. I suggest you learn how to tie and trust them if you tie stuff on car tops.
Just realized something else realted to the length and cinching it down tight.
The roof cross bars will have side to side support adding 2x4s if neccessary.
But if you pull the corners down as tight as you should with rope or ratchets you could risk bowing or bending the frame front to back.
For a guaranteed straight and sqaure haul, add 2x4s lengthwise too.
If you are moving than leave it and buy a new bed frame at your new location. They are not that expensive and it’s better than transporting a wide load.
Have you considered renting a truck for this trip? One large enough to fit it in the bed.
Thanks for the great advice! I was able to transport the bed- car, bed and sanity in tack! I am a happy camper.