Found a great little wheel barrow sifter for compost, should work great at those privy sites when looking for marbles and coins. Im building one for soil around A foundation site. Still working on an idea to furbish strong enough legs to it so all you do is wheel the barrow under it.
Yours looks good...Another idea would be to use a supermarket shopping cart as support for the sifter.
It would be easy to roll in an out of areas. Just an idea...
Take the sifter from your drawing using just the screen box. Use 1/2" pipe to make legs attach them to one end of the screen box add a 2x4" to the other end to keep it from sliding into the wheel barrow. The screen box sits at an angle. To catch the fines hook the 2x4 over the back of the wheel barrow, to catch the heavies hook the 2x4 over the front.
If you need help, drop a PM & I'll shoot you some pics of one of mine.