Today, I just went ahead and made that goddamn hill that I´ve been worrying about for so long. My vacation was canceled by the volcano ashes by the way.
I drew the hill on the new game mat that arrived from Hotz a week ago. Then I cut out the hill with a knife and trimmed the edges with a pair of scissors. I bought two sheets of foamboard, drew the shape of the hill and cut the foamboard. Then I trimmed the edges of the foamboard to create a gentler slope. Then I glued on the mat and cut the whole hill in two pieces for easier storage and transportation. All that remains to be done is finishing the edges of the hill. I´m still experimenting on how to best do that. Probably glue, sand and green with a hint of brown.
All in all: The hill is not perfect but it does the trick.