Hi Lars, here is some more information on how the rectangle solution works, it is probably the most simple method.

To start with I used Edit/Separate to break your shape apart into individual surfaces and then hid all them all except for the one that I wanted to work on:



I went to the side view which had the closest to a straight-on view of your hole and drew a square there:



If your square does not line up well with the hole, you may need to select it and then use Transform/Rotate on it so that it is aligned more evenly with the hole.

So now you have a surface and a square, it looks like this in the 3D view:



You can now select the surface, and run Edit/Trim, then select the square as the cutting object. The surface will be cut by the projection of the square and you can pick which pieces to remove, pick this piece to remove:



Now you have a square hole. However, it is bounded by 5 edges since this crosses a seam edge. Here is shown one of the pieces that was divided in 2:



You can't make a good Network surface out of 5 edges like this, you need to have 4 curves to make a simple Network surface. So you want to select these 2 edges:



Then use Edit/Join to create a new curve object that is made up of these 2 edges joined together.

Now deselect everything by clicking in space or pushing Esc, then select this new joined curve, and the 3 other edges:



Now with those 4 curves (the one joined curve, and then 3 edges of the hole), you can create a Network out of them:



If you can make this square not much larger than the original hole it will be a smoother fit.

- Michael