I am assuming that you are putting shelves in a Standard
Scamp with a
fiberglass closet/cabinet.
If you use a cabinet grade 3/4" plywood, you could drill through the side of the cabinet and screw the shelf in place, using snap caps for cosmetic reasons.
This same method could be used by screwing in 1X2 oak strips as shelf supports and then using luan shelves placed on the strips.
This would allow shelves to be adjustable.
Another method could be as simple as using small machine screws, bolted through the sides and covered with snap caps making "pins" on the inside similar to kitchen cabinet shelves. The threads could be covered with bits of snug fitting tygon tubing.
You could simply glue 1X2 pine strips vertically in the corners, cut to shelf height, place the shelf... then place 4 more strips on top of that shelf and place the next shelf.
You could even place two wood strips all the way up on the back corners and install two shelf brackets similar to the ones below , making them completely adjustable...