[Joe G.]

Unless you have to, don't through-bolt, place your nuts on the inside of the tube, so that you're only putting load on one side of the tube. For this reason, rivets are my preferred fastener for working with this type of material, since they do this automatically, and can do so blind, right in the center of a tube.

Wood or plastic inserts also work well for the ends of tubes. Plastic has the advantage of being tappable.

If this can't be done, use versa block style construction, clamping onto the tube, allowing the strongest part, the edges, to support your parts. This can be done even without advanced machining. Use a few small screws with through holes to locate your part on the tube, and then bigger screws on the outside to provide the real clamping force.

Short of this, make sure that the parts on both sides of the tube are supported by the whole tube, not just the middle. Again, the edges, where you gain the full support of the sides parallel to your fastener, are the strongest parts. Make sure your part takes up the whole height of the tube when possible. A cool trick to achieve this is to make an aluminum plate the height of the tube to slip under your nuts, distributing the load.