Coffee heater rocket mass stove

Its been some time since I have been on this site. In the earlier days I think I got altogether too adventurous! Ive slowed down a bit now. I would love to make my version of the coffee heater rocket stove as shown in Iantos book, and even though basic instructions are given, Im still quite confused as to the dimensions. Are there any clearer details on the dimensions etc does anyone know? My perception is that the heat riser is a square shape, 10 x 10 cms, made of possibly vermiculite board but I could be way off on this! And the whole thing pretty much sealed with cob. There is an area that is described as a vermiculite 'plate' and Im very puzzled about that. Any help would be very much appreciated!
Many thanks