Hi Mike,

Yes, you need to build a panel (ignore the wall, you are right it will most likely be faster and sure enough more dependable since you built it) - offset it 1" or so.

You do not need plywood - just build it as regular stucco wall and apply the layers ontop - the # of layers and thickness of the wall depends on the length of painting time required for the longest/most complex giornata, aka - simple compositions with simple giornatas will require "thinner" wall. You can view detailed diagram of plaster thicknesses in this Fresco Painting DVD - Buon Fresco Foundations: Volume1 - FRESCO PLASTER:

http://www.truefresco.com/frescoshop/product_info.php?products_id=45

You do not need any plywood backing - 10ft is low just nail 2X3 to the wall and stuccco mesh over for the scratch and rough + fresco plaster layers.