i have seen people use wood dowels (the really small kind) and all kinds of other things to get the tweeter where they want. once they have it where they want, reinforce the area so you can stretch fabric over the top. apply resin and a few layers of fiberglass. sand, mud, sand, paint, or flock if you wanna match the interior
but, i am by far NO expert, such repeating what i have seen over the years. i think the tweeter put on the triangle like you have it would look and sound good. maybe a bit bulky?
im used to this kind of stuff
where you would make it sit how you want. then wrap the entire a pillar, and do w/e you want to match the interior (i usually would use flocking)
again, no expert. not by a long shot, and i havent done anything like that in such a long time
Lloyd