Referencing my previous - we fashioned some supports out of wood (obviously) and screwed them onto the board from the backside.
Today I would use glue and fasteners, but alas we all learn over time. These are "miniature" shelves, set at a 45 degree angle with an upward support, one positions behind the trigger and the other on the stock. They are placed so the rifle sits horizontal on the board assembly.
Can take a photo and forward if you would like me to do so.
Al