I'd advise installing batons inside the roof first which all this can be screwed onto. You could perhaps use three lengths of 70 x 22mm 4.8m Treated Pine Decking cut to span the inside width of the roof. Pre-drill holes in the roof at these locations and screw Buildex Roof Zip C4 HexHead Screws M6 x 25 Windspray Bx100 down through the roof into the battens. The reason for suggesting you do this is to not have un-sealed holes in your roof. The Buildex screws have a rubber washer that stops water leaking in.