Re: Need to tie something on the roof...
Get it delivered or put it in a trailer.
It is not worth damaging the car with straps to save a few bucks. The only way aside from roof racks, would be to run a strap around it on the front and rear doors. This however will damage the paint and door rubbers. The mattress is flexible and needs to be tied down properly.
MY2011 CRD Overland in Bright Silver
I Bought an SRT - Bright Silver, HK + Roof