In the image below, you'll see the final rendering of the cabinet I'm building. If you look closely, you'll see the top is designed with edging so that the middle section is supported on all sides. It can also expand and contract because I've allowed space in the tongue and groove joint at the back for expansion.
In the images below, you'll see how my picture framing jig (accurate to within 0.001") saved me a lot of time and improved accuracy when making the 45º cuts for the edging pieces.
the mid section was simply jointed, thicknessed and glued. i used my table saw to cut the grooves in the side pieces which were marked using my marking gauge.