When I built me sluice I put 2 bits of flat across the top wide area, ,my sieve fits in between them and classifies what I shovel in; water runs under it to wash the dirt . I find the material flows better down the riffle if it is a smaller constant size , with no big rocks.
I have 6000 But will not give up my 4500, it has so much more with coil flexibility; I love the lightweight of the 6000 and it is a very sensitive detector. I have picked up buckshot and gold that should have been got with other machines .. Love to see a X coil come out fore it, Pick up and go...
I got a Go find 66 for he grandkids, has different tones for coins, iron and gold ; waterproof to 3 mts.. Bought it second hand for $250 used twice, so in mint condition..
Cheers John