I ended up buying the Severne Dyno 115 L. Haven’t had much time on it yet. I also still need to buy a big single fin so I can try it on flat water with a 7.5 (comes with 24 cm center and 12 cm thrusters, plus blanking plates for the thruster boxes).
Very interested to hear how you get on – the board specs claim it is good with a 40cm single powerbox fin (supplied with 2x12cm and 24cm) that is quite a range of fins assuming those recommended / supplied also allow for some bigger / smaller fins to be used!!!
I guess you would need to be moving your straps about when going from thruster to 40cm single.