Hello everyone,
Based on a suggestion made by @DoubleDAZ I have modified both of the Top Frames to include a longitudinal piece of 1-1/4 x 3/16 Flat Bar in the middle of the open framework. This will cut in half the open unsupported area where the 3/8" Luan Plywood will be. The attached picture is for the left side and will be the same for the right side also.