I built a 4' deep shelf over the garage door by spanning the 21+ feet with a homemade beam made from two 2x10 with 1/2 plywood laminated between them. Added a ledger board to one side. Lots of liquid nails and 16 penny nails.
Beam was 4' in from garage door face. Made 'floor joists' out of 2x4 to go from beam, 16" oc. (resting on ledger board) to wall over garage door. Covered whole thing with 1/2" plywood.
That was 9 years ago. It is totally filled/packed (mostly 'junk') and to this day there is no noticeable sag in the homemade beam/shelf.
Fairly inexpensive, very strong and about 80 sq ft of storage space.
In a previous house I had I did something similar except across the back of the garage and the near edge was suspended from the ceiling joists on 3/4" threaded rod. This was a lower 4' deep shelf that was just high enough to get the car hoods underneath it.
If you do suspend something be sure that if you have ceiling trusses that you don't compromise the integrity of the trusses by hanging too much weight off of them.