Alain Picard, his demo painting and model at IAPS 2015, photo by Carolyn Hancock
In a Uart interview, Alain Picard cited the mounting products he uses to create a stable working surface: Miracle Muck and Scotch 568 Positionable Mounting Adhesive. The Miracle Muck is a glue, messy and involved, so the Scotch 568 is his go-to product. It is purchased by roll and can be cut to size and repositioned. Amazon link for reference.
Other options for mounting
Dry mounted at the local craft store (expensive)
Tacky spray (adhesive separates after applying a wash)