The mounting brackets need a cross-brace at the top and bottom. With double down bars only allow the bag to "flop" around. I don't know how heavy duty the steel is on the hardware I was looking at but if you want something substantial go to a machine shop and have them weld up a bracket set that is stronger and bolts up using your frame rails. The other option is use the existing bars and put them in a X pattern front to back-back to front and a center bolt to hold the two brackets together. As long as the bracket strength is enough to keep the bag (loaded) from pushing toward your tire, it should be OK.
Me, I'd go to a machine shop and get proper brackets made so you can overload the bags. You could then have them polished/chromed/powder-coated/painted, etc. to suit. You saved enough on the bags to do it right. The "free" mounting kit just looks cheesey.