I have a king cab pickup, so my truck bed is short. I use a bed extender to hold my 12' yak. My buddy has a hatchback Mercedes suv thing (GLS maybe?) and he got a bed extender and just opens the hatchback and slides in his yak. Much easier than putting it on the roof. Provided you have a hitch receiver, you could do the same thing.
Not the one I have, but similar. Just wanted to share something with a picture to give you an idea what I was talking about.
I carry my tackle boxes in a milk crate that gets bungled to my yak, so they don't go anywhere.
Haha. I was thinking about getting a very similar bed extender. Do you like it?
Could maybe go for a Bed Extender. They can definitely be found cheaper then this. Personally I'd build a rack like the others have said.