Creating circular covers can be troublesome since the finite element mesh needs to be polygonal. If you still want circular covers in your model there are two workarounds.
1) generate new region by using mesh
2) generate new hole by using solids
1) Mesh option (description in the pictures):
1) generate new region by using mesh
2) generate new hole by using solids
1) Mesh option (description in the pictures):
2) Solid option (description in the pictures):
