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Hi Steve
You can use Plaster of Paris or cement. I often mix cement with plaster. The plaster helps it dry a bit quicker. If you are in a hurry casting plaster might be an option. For an F9, a flat surface is probably ok as the curve is so shallow. So a flat piece of thicker glass, metal etc would be ok. Normally you use the oversized tool for polishing not grinding. If it passed the sharpie / pencil test it’s probably fine though.
Personally I would use a smaller lap. I think others have used something your size successfully but as I said I found it hard to get good contact, leading to a rough surface. So I would try about 206 mm. Plywood might be ok but it would need to be very well sealed.
Are you referring to Texerau’s method of making individual pitch squares and attaching them to the tool? You can do it that way but it seems like a lot of extra time, work and more pressing than other methods. I don’t see any advantage in the extra effort. It just looks neater which is unnecessary. Gordon Waite has some good videos, one of them is on making a pitch lap. I like to use a mould to make mine eg. a piece of a laundry basket with suitable size square holes. I cover it with soapy cerium oxide and water then press it into the warm pitch to make the grid pattern.
Mel uses more than one pitch lap because he makes ultra fast mirrors - sub F3. For an F9 one lap is all you need.
Rod.
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