Hi Gunter, or another way is join up these 3 segments at each corner:
Select the 4 corner curves:
Run Construct > Loft and set loft options for "Loft style = Straight" and turn on the "Closed" checkbox:
Then the top is planar so you can finish it by selecting the object and run Construct > Planar, it will fill in the top.
- Michael
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