Picture of Adjustable tree saver from seat belts (sewing optional).
This Instructable will present a way to make a tree saver from a seat belt. I tried to come up with a way that did not require sewing. I will add sewing to this Instructable at a later time as an alternative to tying belts. 

A tree saver is a wide belt or strap that is used to tie onto a tree. Due to its width it will not dig into the bark of the tree when pressure is applied to it.

Seat belts are ideal for tying up hammocks since they are wide and will not stretch when weight is applied to them.

Screwdrivers (to remove car panels)
Wrenches      (to detach seat belts)
Hack saw      (to remove hardware on belt)
Scissors        (for trimming belt)
Lighter           (to seal edges)

Quick links     (to create tightening loops)
Seat belts.

Optional Tools and materials:
Fishing line, 20lb test, (for sewing belts together)

Miscellaneous information  :
From what I have researched online, a seat belt will have a tensile strength of greater than 3000lbs, some are more like 6000lbs. Reducing the width of the belt will not effect it's ability to support your weight.
Seat belts are sometimes made of polyester and sometimes of nylon. The newer belts are polyester. The longer belts I have are from a vehicle that was 25 years old. I noticed that they have a tendency to fray easier than the newer belts.