The optional tool tray is added as part of the apron. If the tool tray is not wanted, continue adding the apron around the bench. If the tool tray is to be added, extend the end aprons the width of the tool tray.
With the tool tray, the back aprons do not need to be 8/4 material. I used some 4/4 Cherry I had on hand for the both sides of the tool tray. The width of the tool tray was dictated by a piece of scrap 3/8 shop ply. The end aprons were inlet for the back apron, a grove cut around the bottom of the end aprons, the back apron, and the back of the tool tray.
bHere is the grove cut on the end apron to inlet the edging of the back of the bench into the end apron. Both ends are inlet.
The bottom to the tool tray is grooved to set the tray bottom.
One end apron is set in place using a spine, dowels, or biscuits. The rear apron is set in place with dowels in the joint at the end aprons. The remaining end apron is set in place next. Slide the tool tray bottom in and set the back of the tool tray in place using dowels at the ends where they are joined to the end aprons.
The tool tray is in place and in use.