If you put a wooden dowel inside the tube before crushing it, you avoid the dimple in the flat surface, except at the very ends.

There the wood extrudes out the end and does not provide the support needed. Just allow an extra inch on each end to trim off the malformed portion. Place the tube in a wood stove to burn out the wood, or you can heat the tube up with flame. Eventually the wood will char enough that it slides out. Be careful, as it slides out and encounters air it burst into a flaming stick. Rather spectacular.
-Doug