The problem with the Rola rack is that its's too narrow. So, if you fab brackets similar to the defender ones, there will be a lot of stress on those brackets (especially when the rack is loaded up). You could make a crossrail that "hugs" the roof and bolts to the factory rails, then bolt the rack to it.

I'm having the same issue but, I plan on widening my rack so it will almost sit on the factory rails. Then I'll fab some brackets for it and install the lights. Good luck.