Punch plates are often used at the top of the sluice box usually over UR in a highbanker.
If it's a dredge, different story.
As the water falls it has no "horizontal flow" therefore the area really doesn't "exchange" like the rest of the box.
Remember... depending on the design, the water often falls straight DOWN here.
Placing 1/4" punch held about 1/2 - 3/4" over the matting choice, including moss or whatever you use, will allow a quiet
capture zone similar to a quasi fluid bed, but not allow larger rocks to simply sit there and build up.
It's a lot like the theory of a slick plate, only you are capturing gold at the same time.
This method works REALLY well if you have a crash box style dredge as well, which often suffers from
LARGE rocks piling up in the crash box.
Hope I built the VISUAL well enough there.
Doc