The Reptile V550 (v2) is a cheap frame, so it does not have any instructions. Luckily the PDB just fits enough the top frame after a few trial and error of using different screws and mounts. I used a M3 screw and a nut, I also eventually lowered it a bit later from this photo to accommodate the gimbal board.
Also pictured here is side by side comparison with my current DJI F450 arms. The cheaper one is not as white and narrower, it seems to be just as stiff though.