To start, here are a couple of views of the alternative layout of the boards. I can't lay them all out as I don't have an eight foot wide table!
Today I added some texture to the boards. I decided to try some plaster cloth since I picked some up on clearance from Michaels. It comes as a roll of mesh impregnated with plaster. To use, cut into strips, dip in water and then lay on the table. It's pretty messy, but easy enough to do.
I started with one board, completely layered it with plaster, and moved on to the second. This is how much I was able to cover with one pack, roughly 20 cm by 3.5 m. When I ran out, I went back to Michaels and picked up the last two packs they had in stock.
This is as far as I got with the total of three packs. It didn't quite cover the entire surface. The main thing is that I covered over the plywood base and all the joints for the insulation board.
The mesh pattern is visible in the plastered areas. In future steps I'll cover this up with more texturing, with sand, gravel etc.