Hey DC,
No reason to put it on concrete, it was just the smoothest flattest thing around. Plywood would work too, Maybe put some wax paper or parchment in between.
I wouldn't use OSB/chip board, the chips might make indents.
I find sandpaper doesn't help shine the bark. A scraper works best. I scrape it down after glue up, in case glue seeps through a lenticil and makes a mess. Don't want to be scraping it too thin in that case.