Hi Lars, I think that Patch is really your best bet...

You can fill in that area in MoI by dividing things up into smaller pieces, but it will be difficult to get the kind of internal continuous result that Patch will give you.

But if you want to do it in MoI, one dividing method would be to create a polyline or curve in the center region with 4 corners like this:



Now trim the curves where that center piece touches them, which will form 4-sided regions like this:



Then you can select the curves for each 4-sided region and fill them in with the Network command:



But I think you'll get a more pleasing result using Patch.

- Michael