To put a single rail at the bottom of a cabinet, the existing bottom panel needs to be receded. In case there is no bottom panel, add one by selecting the cabinet, going to the Construction icon at the top of the design panel, and under the Carcass drop-down menu, switch on the Bottom toggle under the Carcass Panels Visibility menu.

Then, click on the Carcass Panels Extension drop-down and select Bottom Panel tab. Then, go to the Depth Extension menu, and switch to Extension Parameters .

Depending on the edge of the panel you wish to recede to make a rail, type in the negative value of the desired recession distance into the Bottom field (to recede from the front edge of the panel) or Top field (to recede from the back edge of the panel).