Hi Tess The meshing errors are caused by imprecise geometry. I recommend you open the geometry in SpaceClaim. Copy/paste the edges on one side to create curves. Delete the faces to get rid of the slots, then extrude the curves to make new slots. You might want to do a repair operation on the curves before you use them to make a new slot.
If you need help applying this advice, create a Workbench Archive file of the Geometry and I will try it.
Kind regards Peter