Hi Ed!

You need to use washed river sand, silica sand contains mica and will eventually separate and fall off.(mica - any of various colored or transparent mineral silicates crystallizing in monoclinic forms that readily separate into very thin leaves)
To obtain fine sand for the final layer - sift washed riversand through fine screen (window screen works). Also sand must be free from loam, clay, mica, and gypsum.

About your trays - good idea, but how do you plan to get the boards out? (mortar/plaster will stick to it good) you will have to, perhaps, line them up with plastic?. Also you need to use wire mash of some kind - for small panels "expanded metal" is good, in the scratch coat as the support for plaster. Plaster needs support - it is too brittle on its own. I would suggest you use the system posted by Gary in the topic "whether it is necessary to add c..."

Good Luck!