The designer recommends setting this servo box into the fuselage top here. The triangle behind is the tail fin strake. The idea is that the servo fits into the box, with the servo arms just clear of the tail. The strake is then just tacked on over the top of the servo arms, with a small slot cut into it to give clearance. Problem is that if you ever need to get to the servo, you need to take the strake off first. I don't like this.