Depending on where the sidefins are placed, the main issue with a 2+1 is the center box often being up too far to fit the standard ‘thruster’ formula. A longbox can also affect tail flex.
I started looking for small single fins with the screw tab in front and cutting away at the back of the base so it could overlap the back of the box.
I think Rainbow fins makes a rear fin like John explained. Well I know they do since I have one, but not with the rear cut away. I think they do it with the cut away, too. That looks like a fun board. Mike