Seed cleaning pans are very expensive and not big enough to do much volume very quickly. This type of approach can be done with all food-grade parts and with the flour sifting I use it for lets me easily do several pounds per minute.
Parts
*Commercial kitchen type of 18"x26" polypropylene tub. Shown here is a 1/4 height tub at 3" high, but you can get them in 1/2 height (6") and full height (12").
*Screen or mesh: Besides all the standard hardware store options, you can order mesh of all sizes online. Special seed cleaning screen material might also be available.
*Strong but thin thread/twine
Tools
*Utility Knife
*Hand saw
*Heavy duty manual sewing needle
*Wire cutters, snipping shears, etc.
*Drill and bit
Steps
1. Plan out how you will attach your screen to the tub and plan your cut accordingly.
2. Cut out the bottom of tub or build your own open-bottom box. In the polypropylene tub it is easy to start the cut with a utility knife and then finish with a hand-saw.
3. Based on the size of your needle and thread/twine, drill holes all around the edge of the tub.
4. File/sand the tub to clean everything up.
5. Cut the screen/mesh to size.
6. Sew on the screen to the base of the tub.
Use
A gentle eccentric shaking motion does a surprisingly fast job of dropping all the heavier items through the screen and leaving the big and lightweight items on the top. Screens can be stacked, but it can make it hard to know when to stop shaking since you can only see the progress on the top screen. For complete cleaning, this technique would be typically be supplemented with some form of air-based winnowing.
