When I did my Ibiza, Autoleads didnt make adaptors, so I got some 18mm MDF and fitted my speakers on custom mounts.
It took a lot of maths as space was limited, and I've not touched my MK5 so I would advise looking at speaker dimensions and most importantly, review the amount of space you have between the door card and the window regulator plate, and the space between the other side of that plate and the window when its fully down. The last thing you want to do is smash 2 side windows on the magnets, or crush your new speakers screwing the door card back on.