It's a slight bend in all the verticals mate, bloody pain in the arse to get just right as once you bend it you can't go back. Make sure it has very strong welds as there is a lot of stress going through them especially where the fwd sloping rear strut meets the aft sloping top strut. That join is critical and you need to make the join as strong as the rest of the pipe to help prevent fatigue cracking and if it does start cracking you may have to put a small gusset in there to stiffen it up a bit. I made some gussets up but they looked crap, I found the clean look to be better and am taking the chance with the cracking.
I have found that the tower can can get some lateral resonance when going through continuous chop and I will be probably be putting some hand holds to act as gussets up in the top corners, going from the rear lateral member to the rear vertical strut to help with stiffness (see pic). I will manufacture the bimini before I do the gussets so they don't end up clashing, I may even incorporate the bimini's frame into the tower to add rigidity and cut the gussets altogether.