If you have a router, but the frame together with pocket screws then run a 1/4" groove around the inside of the frame with a rabbit bit.

If you dont have a router use 3/4" stock for the frame. Set the table saw blade to 1/2 the height of the stock apx 3/8". Then set you fence on 3/8" . Run your frame stock flat then on the edge to create a rabbit. after you run each piece miter the edges and put it together with bisquits or pocket screws.