When there are places to tie the ropes to, or to hook the bungee cords to , then it is easy to tie loads to car roofs with ropes or bungee cords
The places to tie the ropes, or hook the bungee cords , are w called anchor points.
Here is how to make these temporary anchor points
The use of a ratchet strap is also most highly recommended.
The use of a rubbery roof pad is also highly recomended
Step 1: Materials
The materials have to be strong and thin.
For my vehicle, I would consider strong as having a working load of 75 pounds or more
I would consider thin to be less than 3/8 inches thick.
Although I will illustrate the procedure with nylon military belts ( straps)
Other materials could have been used.
Cable Ties ( Zip Ties)
Other thin wirres or cables
Step 2: Tie Straps Around Center Posts of Cars
Turn the knots inside the car and close the doors
The knot used to tie the ends of the strap together is called a "water knot"
Many sites on the web show how to tie a water knot.
If tying this knot is a problem, then consider using cable ties or paracord or a wire or cable.
Small clamps will bind cables and can be obtained at the Ace Hardware store.
Step 7: Option: Instead of Straps, Use Long Cable Ties
These are also known as Zip ties
They are thin and strong.
They can cost from 0.30 - 1.00 each and are not re-useable
But they are easy.
Step 8: Details of Cord Tied To Anchors
"Round Turn and Two Half Hitches"