Thank you !
Had to stop last night and had some direction for cutting the original case now.
I know that when using acrylic cement, the plastic melts some when squishing the pieces together which is good for bonding. BUT as the material squishes I could possibly lose some dimension to the repair and could make it look crooked at all kinds of angles after its stuck.
I decided to slice the original case so I would end up with MORE material than needed so I could sand it down a little at a time to get it just right or hope the plastic would squish just enough. Remember with acrylic cement, if there is no material to squish together, it will not fill gaps/voids so for a strong bond the 2 parts must be touching at every mating surface.
I used the thickness of this thin metal bracket I had laying around, which happened to have a 90* bend and made a perfect jig for offsetting my cut and also to set the perpendicular slice I want to make next.