To attach the shelf, measure and mark the point that’s 16" from the top inside edge of the frame. Place the shelf on the point and draw a line where the top and bottom of the where shelf will set.
Remove shelf and mark a point in the center. Then mark another point 1-1/2" from each stile in the middle of where the shelf will set. Drill a 1/8" hole through the sheet at the (3) marked points. Flip the board over and drill countersink holes into the same 1/8" holes.