Your custom drawer insert will be sized slightly smaller than your drawer's dimensions.
The insert's width will be 1/8" less. This will leave 1/16" of space between the insert and both sides of your drawer.
The insert's depth will be 1/8" less, leaving 1/16" of space in the front and back, UNLESS you have hardware screws protruding at the front of your drawer. In that case, the insert's depth will be 1/4" less than the drawer's depth, leaving 1/16" in the back and 3/16" of space in the front. Hardware screws should not stick out more than 1/8" (the thickness of two pennies).