I've looked at this a few times... and for the first time I realized that you machined the concrete?! Impressive!!
Since I wanted a perfect surface, I purchased a sheet of ¾” melamine for the Face Side of the form. I used six bags of High PSI concrete I got at HomeDepot costing about $6.50 per bag. The type they can use for concrete countertops. I also used some wire mesh to make sure the concrete didn’t crack. I screed and troweled the cement making it as level as I could. This would be the Back Side, however, after it set, I had high areas. I let it cure for two days before I purchased a 4” diamond grinding wheel from Harbor Freight for $20, machined the end of a 5/8” bolt on my lathe so it would fit into a ½” collet. I ended up milling about 6 mm, mostly around the edges to make each section exactly the same thickness. It took about 1-hour per section to grind. It also took me about 3-hours to clean all the concrete dust out of my shop. Glad I had a leaf blower and a lot of fans. Thanks for looking.