So I created a solid part that features a series of holes inside a part with fillet edges. I then offset the hole and thickened it to create a part that can rotate inside the larger one. How do I fill in this hole? Tried knitting, filling in the surface (none work), and when thickening it I don't get the option to "create solid from enclosed volume", which would be ideal.

(trying to completely fill in the red hollow rotating piece in the middle)