Here is an inexpensive easy way to make the shelf. Material needed: 4 each 1x2 for the rails, plywood for the shelf and Harbor Freight ball bearing rollers.

Position bottom rails at desired position, screw rollers to the bottom of shelf. Then position top rail above shelf with a 1/8" gap. Add a stop block at the desired shelf extension. The 5/8" size roller should roll perfectly on the 3/4" edge of the rail.

I built a storage building solo. I made brackets using these rollers to help me set the roof trusses. Worked like a champ. Handled the weight with no problem. Good luck.