+1 to what Pastorbob suggested. That bag support thing was made by Park (iIrc) and came in two sizes. It's a great design, very strong, and when used correctly very stable. But you have to get lucky with the fit. I custom made a bag (basically a copy of the one Pastorbob has) to fit my frame and saddle height. If you know what you're looking for, you'll see the same bag support in this photo:
