Hi potan, was that object constructed inside of MoI or is it imported from some other CAD program?

MoI does not really have a good tool currently for filling in an area like that. You might try using the "Patch" command in Rhino on it, but first it would probably help to untrim and retrim the fillets so they don't have an awkward wavy shape at their end like they currently do. I've done that on the attached model so that the area to fill does not have such an irregular outline as it did before. Currently I've just filled it in using the Construct > Network command in MoI which does not guarantee continuity but I think the overall shape is much closer to what you're looking for. Try using the Patch command in Rhino to fill it in instead and see if that's better.

- Michael