Joe, a lot depends upon the loads you are going to be carrying up in the attic both on the floor and roof for live and dead loads and how much snow you need to hold for your area of the country. I assume you want to use it for storage and not for living. Most builders I know don't design trusses anymore or build them for that matter. Go to your nearest lumber yard that builds trusses and they will fix you up with what you want. This isn't the place to try to save a buck and anyway most are very competitive. If all else fails, get an architect to draw it up.
Looking at your picture, I'm not sure your diagonal brace alone will do the job depending upon what the loads are. Trusses also help to keep the building from falling in from snow or racking over time. Build it to last. I've seen a lot of old garages (and no used loft area) that just had a 2by4 run up to the ridge and spaced on 24" centers with a long 2by4 spanning both wall to hold it together every 6 or 8 feet. They all seem to develop a 20 degree list to port or starboard over the years until the fall in.