If you add a construction line between the two points in the slot, you can the use that lines midpoint to create a horizontal constraint between the origin.
To add this constraint, you right click your construction line...and hit "Select Midpoint"...then while holding control down, select your origin and add a horizontal relation.
Hope this helps,
It is really helpful when designing parts in SW to sit down ahead of time and think a little bit about how the part could best be built. If the part is going to be symmetrical, then make one of the base planes the plane of symmetry. If it also is going to have elements that are symmetrical about another plane, think about making that plane one of the other base planes. It's the just the same as putting the first lines down on paper in the old days; you didn't just put stuff down any old place, you thought about what the drawing was going to look like when finished.