With a large format tile, it isn't as big a problem. I'd mush a bunch of thinset in there so you don't have a cavity - try to coat the edge of the cbu so it will stick. Note, you should use the reinforcement tape on the seams and corners. This is a mesh tape, but make sure you get the stuff designed for cement work - it looks like the same stuff you use on drywall, but isn't. You won't be walking on the walls, so 100% support isn't as critical.
While some people grout corners, the industry standard is to caulk all changes of plane. You can buy color matched caulk in both sanded and unsanded consistencies.
Next time, measure twice, cut once!
For the best help with tiling questions, try www.johnbridge.com.