1 buy a door new or used at an auction.
Or
2 build one out of plate.
To mount it I think that I would use concrete anchors but I would use the epoxy type. That way you have no problems with moisture getting in around the anchor.
I also thought of making a fire box out of brick for a RF in a built in out-door kitchen, but have decided my brick laying skills are not up to the task.
When I was thinking through how I would build the doors I had planned on building a frame out of 2 or 3 inch angle iron and brick around it to keep it in place. Then you can hinge the doors on the angle iron. Also on the inside of the angle iron you could weld tabs that can be placed in-between the bricks in the mortar.
For that size I think I would use 2 doors 24X24 that open up so you have access to the whole 48, and I think it would look better than 1 door that is longer.