On my car, i have 2 tow hooks under the front bumper . Thats where I attach them to. On the back I just attach it to the plastic lip on the bumper... I havent lost the bumper yet. I would just look for anything solid under the front of your car to attach them too. You dont need them that tight, just tight enough that there is no slack.
I bought two nylon straps that attach inside the hood. They are bolted to the inside of the hood and you tie your straps to them. When not in use they are hidden under the hood. Only about a foot long.
I had the straps under the hood on my old car (well, not old...got into an accident and totalled my 2007 toyota highlander...not my fault) and I got a new 2008 highlander, which has a bit of a different body style...I guess I will search under the hood for some new bolts to connect to...
Rutabaga sells the under the hoods straps. I might try making a set with some extra nylon strapping I have and some grommets from the hardware store.
If you have a sit-in kayak, you can use the combing to your advantage also. I have used two straps to hold the boat down, then used two additional straps to go up and around the combing and tie that back to the roof rack. The additional straps keep the boat from slide back and forth.