Hi Markli, is your battery a tough fit on the EVO. The location and pressure of where you grip the EVO just may just be a factor.
When I grip the base i notice that my 3rd finger and pinkie is pushing on the vent bars. Is it possible that the one vent bar that is missing had broke off and has created a weakness in that area.
It just may be that the missing vent bar has caused enough weakness in that area to cause the crack.
Also, when held in the correct light is easily seen, I notice there is some concave malformation approx ) 4-5 mm long on each side of the body, just to the right of each vent. I,m sure it's on all EVOs.
It's called shrinkage in the plastics molding world. Shrinkage plays a role in producing weak spots on plastics and on alloy moldings (e.g.wheels) being similar processes.
So if it hasn't been banged, dropped, prodded, shot at, or clawed by an eagle, then the above may be a factor.
Did you ever find the missing bar ? If it's in your carrier it may give you a clue.
Also, i would not attempt to glue back those thin edges. would not last long. I would stitch glue them first with a piece of similar plastic, either from the inside or outside, then let it heal before filling the whole crack with glue.