I have a 7'2'' with a tri fin set up. The fins have the two square male ends that fit into the 2 female slots (normal tri fin set up). The board rides so much like a longboard though that I really wanna try out a single fin set up on it. I haven't seen any single fins that'll fit in the center slot... can this be done?
Gonna be hard to find a 7+ inch fin that has FCS (the style you described) ends. Not saying they aren't out there. Use Google. My 7'2" single fin pin uses a 7 inch Rainbow Rake. Extreme option is to have a shaper cut out the center FCS box and make a slot for a single fin box, then you can ride 2+1 with small side bites.
Sounds like the search for the holy grail. I'm thinking you may want to cut out a fin box. But there has to be a reason I haven't seen it done on a shortboard in the last fifteen years. Probably turns like a dump truck. The Campbell Bros. do bonzers with a fin box, but that is for Hawaiian juice, not the stuff we call waves. But on those epic days with late air drops into blind offshore spit, it sounds valid.