There is some confusion here. Maybe I could take a photo of a recent home-made tile and paste it into a reply message. You could see better what I'm trying to say . What's a good file-size for images, and how do I do it?
I remember when visiting your workshop with you last summer that you had some masonite panels with burlap glued to them and one thin coat of intonaco layer on them for painting. Very light-weight. You said that they were simple but effective supports, although the painting time is limited ( three hours approximately)so they were more for practice. So what I'm trying to do Ilia , is make something thinner and light-weight like that, but permanent, made with cement, lime, sand, etc. for more painting time. I need at least 6 hours of painting time, hopefully more. 8 hours is best; after that I get too tired to see what I'm doing. So, I try to make a support that's 1/8 inch thick, with metal lath and cement-sand-structolite, then put two layers of lime putty on it...one rough layer and one intonaco layer. A more solid version of your masonite panels, but not as heavy as a ceramic tile.