6 inches between the rails is a good fit on a 20-inch tube, with the top centerline of the tube down the middle. I'd guess you'll mount the NRS LowPro fittings on the rails, so you can adjust the distance between them pretty easily while the overall length is fixed.
If you've got 1 or 2 crossmembers (for seat & gear bays) between the inner rails, you shouldn't need further crossbraces between the side rails.
But one cheap dodge is to buy some NRS U-bolts (used in the LowePro joints and oarstands) and mount plywood decks between the rails, that give you a place to step and also provide bracing.
Here are some wood decks on a steel double-rail frame (with corners from Cambridge Welding). I use cheap u-bolts from the local hardware, but the NRS version fits their aluminum tubing. Use washers, countersunk into the wood, and acorn nuts to cover the threads.