Punch plates... better know as perf metal... are great to utilize in various areas.
Hoser is correct about the various types and using the right one in the right place is important.
You need to learn about them BEFORE buying them.
Hole sizes... hole spacing... inline or staggered... thickness of metal... type of metal.... etc...
Lot's to learn. Mcmaster Carr is a great place to see the amount of varying types.
We place small bolts in ours to hold them off our mats slightly on our larger dredges.
We vary that bolt size depending on the matting and water flow desired.
On small sluices we can just bend over the sides and make "legs".
When using under a header box where water FALLS on it... you'll have a few rocks plug up the holes.
Simply push these out from the BOTTOM up when you clean out.
When water runs ACROSS the perf, seldom will you have them in it.
In our large dredges we sometimes run two types.
First we use a LESS open plate up top where it is REALLY violent...
Then a REAL OPEN one slightly further down.
Then we go to aggressive bare mats for nuggets or large flakes that might not make it through.
About 80% of our gold is caught in the first 24" of our larger sluices under that plate.
Always have a portion of your sluice that still has riffles or aggressive exchange areas that are NOT covered.
Otherwise when this guy goes through... you'll miss him.
We found him in June I think it was.
