My opinion,
If you have a center console,
Floor and console mount to some degree, or to the gunnels (saw some cool ones in the tropics), but from two different mounting locations for strength.
Welded aluminum pipe, maybe with canvas?
Shouldn't affect towing or navigating unless we are thinking of totally different ideas of t-top, although for navigating it would assist in mounting cool antennas or scanners...
As for wind effect, if the "roof" of canvas/good material is pretty much parallel to the water line and not too big, it won't be terrible.
If you're a new member, I'd search Sculpins posts for ideas, as well as Clintr and albernifishers. Check out Wolfs members boats thread for other ideas too.
Oh, and maybe move this thread to boats and rigs instead of general...