Jim,
The corner pieces you need are available from AMES Performance.
They sell a kit that has the upper and lower corners and the straight pieces for the sides and top edges. It is part number A340.
The corners are available by themselves as pairs. The upper corners are A339P , lower corners are A339S.
There is also a plastic piece that is suppose to be on each end of the cardboard package tray. It is often overlooked. The part number for these is A321.
Loren