If you want to see how the Fliteboard adapter plate builds come together @Kian has a detailed build thread: Frankenstein build (Fliteboard, Gong V2 mast, Flipsky 75200 VESC and in my case a Tritonfoils X1 monofoil).
@Kian does that box have an aluminum insert or metal structure to screw the plate into? Quite a large print!