Thats a classifier william.
http://miningold.com/class.html
get some 2 by 4's to make the box frame 2' squared. and get some metal screen and fasten the screen to the bottom of the box. you could suspend the box over an A frame by some rope to make it easy to shake, or you could put it on top of 2 sawhorses and shake it back and forth.
http://www.twpinc.com/twpinc/control...4X004D0250W48T thats the smallest screen you would want to use for what you are planning.
You can also make a small version by getting a plastic plant pot and cut the base off from about 4 inches from the top. discard or use the base for plants... and staple or fasten with small bolts wire mesh to the bottom of the part you kept. for gold panning this is good to do because you can make several with different mesh from 1/2 inch holes to window screen to reduce the material and remove rocks or other debris. plus they are very portable and easy to use. I do suggest using the top of the plant pot because usualy it has a rim that makes it easier to handle.