The recovery may be improved by laying 1/4 inch screen on top of at least part of the matting. The addition of a few drops of a surfactant (like Jet-Dry, a dishwasher rinse agent) to the recirculating water should help keep the little guys from floating through on the surface tension. These things work best if the material is classified through a 20 mesh screen. Window screen would work. It's pretty easy to pan the stuff that won't go through the screen. Also, the material should not be dry.
Bob Oswin in Edmonton, Alberta contributed this drawing and the associated text file. It requires bending an aluminum trough which you can have done by a sheet metal shop or you can try to do it yourself. See the instructions for the mini-highbanker plans for suggestions on bending aluminum. This design should work better than the rain gutter design below because the bottom is flatter and wider. I also like the hopper and spray bar.
Rain gutter sluice for black sand