After first test the Ikea mat wasn't long enough and I ended up "stitching" two bits together with paracord. Looks very shoddy but actually held up fine and was quite stable when cinched down. I think it would be better if it was ~50% bigger or just with a smaller dry bag.
I had some success copying @Tijmen 's plan (stanley knife not laser cutter)
https://www.lfgss.com/comments/14983630/