Thickness??? do you mean the thickness of the roof itself? not easy to answer unless someone has taken on out and measured it, you could ask Adria, good luck with that though.
The only difference is the colour of the L shaped clamps to fit it with and they tend to have a fair bit of leeway, black is the most common, but my last van had blue, depending on where you order from, I'd go with black and buy the blue too with a condition of returning the unused still in wrapper ones as it will be obvious to them you won't need both sizes.
Actually now my brain has kicked in.
There is a way to do it, but explaining it might confuse you (and me) you will need some stiffish cardboard strips, and double sided tape or Sellotape to make a square C shape, jsut poke it through the Heki opening, and so long as the red and yellow sections stay static when stuck to the main blue C part and are touching inside an out, when you bring it back in and lay it flat, you will have an accurate enough measurement to order the right coloured clamps