I dig the outline. I am making something similar and I went with single, to double to vee out the back. With the vee out the back you can have a curvier rail rocker line and a flatter stringer rocker. And with that wide tail it seems like you would want to move some of it out of the way with vee.
But the board is really short, so maybe turning won’t be an issue and you will want more lift, so a single concave throughout might work.
I’d stay away from the concave idea… it’s flat enough already. I’d go with flat to heavy panel or rolled vee with a hard, tucked edge from nose to tail. I’d start the vee at the wide point and get heavier and heavier through the fins. then I’d go flat behind the back set of fins to loosen it up a bit. These bottoms work great in small mushy surf. You could hull the entire bottom, but then you need better waves and strong paddling arms!
In terms of fin configs… you can add a ton of lift with only 6 or so degrees of cant in the front and around 4 in the back with good sized fins… full bases and medium tips. If the fins are too small, you won’t get the drive or the lift that quads are famous for. I’d toe them in to an inch off the nose and have them both point to the same spot. In terms of placement, I’ve been loving the McKee formula. It’s a bit different from most quads you see, but I’m tellin’ ya… they ride great. You can find it here:
this one’s a 5’11. flat to vee out the tail. EPS/Epoxy. Very flat rocker with a flat deck and thick (2 3/4"). your diamond tail looks crazy fun. let us know how it goes.