I've just finished putting together this little pit for mammoth hunting. I neglected to take any WIP photos, but it was made by cutting a cork-backed table mat to the right shape and scraping out the cork in the center to make a shallow depression. The cork scrapings were then glued back onto the remaining cork around the depression to build it up slightly, giving the impression of depth.
Sand and glue was mixed together into a putty to even up the edged of the the piece and it was all painted brown before being covered by another layer of patio sand and flock. The depression was then painted a mix of black and brown, lightening towards the middle of one side to give a suggestion of some light hitting the base of the pit. Not a masterpiece, but more effective than a plain circle of black card.