John, Great project to test this stuff. You can glue the front of the bottom and allow it to expand to the rear, or more commonly, drive a screw in through the bottom, to the rear drawer wall. You need to make the hole in the bottom a slot and put the screw in to allow for that movement. Or, you can do as I often do... use plywood and just let it float inside four grooves. Also, I want to advise you to test fit often. It astounds me how a single swipe of the plane can remove enough to dramatically change the slop in the fit. Less is more. Like the barber always says... "it's easy to take more off".