I built forms for the back and front of the oven using plywood and melamine. I quartered up some foam pool noodles and hot glued them to the forms to create rounded internal corners. I wasnt sure whether just butting the two oven pieces together and mortaring would be strong enough for the finished oven, so for the molds, I created a overlap/underlap joint, similar to how old sewer pipes join together. This was a little tricky, I'm sure there was an easier/better way.

Overall internal dimensions of the oven will be 23" wide, 28" long, and about 16" high. It's basically a half-barrel, with an extra 4" of wall height added to the bottom.