Fireplace framing question

i have built a frame in my living room for an Ethenol fireplace. I will be using a product called Ardex feather finish to give the exterior the look of poured concrete. I need to know what to face the frame with. I want to make sure it is strong enough not to flex to not have to worry about the concrete cracking down the road. I am thinking 1/2” plywood over the framing then 1/4” hardi backer over the plywood. I would apply the feather finish directly to the hardi backer using mesh tape over the seams. Should I put a bed of mortar between the plywood and cement board like you would on a floor or should i just use screws? Let me know what you think. Thanks