I don't normally jump into these but hey we could actually help with that, have 2 bolt-on solutions actually that might work for you.
First would be to add our multi brackets & tube clamps, 4 of each, then repurpose some extended length Yakima bars you may already own, or could source locally The multi brackets have slots on the bottom that bolt up to your extrusions, and the curves on the top of them and tube clamps are sized for Yakima bars. They were made for this application.
Multi Brackets — Gamiviti - http://www.gamiviti.com/multi-brackets
Second would be we could weld you a pair of cross bars at any length you desire- would be round steel, and they land on a base plate with a 3" hole pattern. You could then add our extrusion adapter plates to get those to mount to your extrusions.
Cross Bars — Gamiviti - http://www.gamiviti.com/cross-bars and
Adapter plates — Gamiviti - http://www.gamiviti.com/adapter-plates.
Actually a third option would be to just purchase some longer extrusions from 80/20, then figure out a way to bolt them to your current extrusions. You could get those joiner plates they sell and run them vertically to tie them together. hope that helps!