hi al! with a couple of hours of peace and quiet (the wife was at work), i managed to cobble this together from the drawings and dimensions that you emailed me. i had to take a little bit of liberty with the design, for structural reasons. there are 16 rather than 18 cross members, on 18.25" centers, i think it would look a bit crowded with one more cross member in each. i also took a bit of liberty on how the metal roof would be attached. the original 48" square would have been too small to be practical, given the over all size of the structure. i doubled the size to 8 feet square, and placed it on it's own set of cross members, keeping the 24" extra outside of the front and back of the roof. i also added the dimensions to the sketchup drawing as well. i hope this will help you out.
i have emailed you the sketchup file, as it was too big to send here.