I used the (almost verz nice) geometry from pjotrx and changed the middle section. I recognised that the middle section had 3 colums which were not necessary as only 2 do also work. The advantage of this modified design is that you save around 25g of material (old: 104g / new: 80g)!
There were also some minor edges on flat surfaces which I removed to have a nicer finish.
My CAD tool somehow messed up the dimensions, so you have to scale it x10 to have the right size. But this is easy to do using Cura.