By the way: I´m using MDF for the front because it already has a very plane surface and no wooden structure at all. You could use plywood as well but if you want to get rid of the wooden structure you´ll have to fill and sand the whole surface various times otherwise it´ll look like wood no matter how much paint you apply.
Did I say artificial leather before? What a pain in the a**!
I first used spray adhesive to glue things together. As expected, straight edges / surfaces are not a big deal, but the round ones are. I started in the round area and worked my way from there. Beside the glue I used a compress air nail gun to attach the leather on the non-visible side to give it a better hold. It needs quite some force to smooth out the edges. With four additional clamps, a small scraper and a lot of swearing and sticky fingers I managed to at least let the visible side look good.
Meanwhile the GS is also covered and it was way easier. Instead of spray adhesive, I used double-sided adhesive tape. Now even the round parts are looking really cool. I wouldn´t recommend to use spray adhesive. Go for some good quality (carpet) double-sided adhesive tape and you´ll be surprised how fast easy it is.