All most all sluice boxes use a carpet or some form of ribbed matting or miners moss as the first trap in the sluice box. The second trap may be expanded metal or a riffle system that goes over the top of the first trap and some might go expanded metal over the first trap with a riffle system over the expanded metal giving the sluice box 3 traps to stop and hold the gold.
The design and style of the riffle system is very important, for if the riffles are spaced to close or to far away from each other this can and will effect the performance of the riffles to capture gold. Also the size and shape of the riffles are just as important as the spacing is.
I have never bought a sluice box, all of my boxes are homemade and field tested with several different styles and designs of riffle systems tested over the last 40 years. Boxes can be designed and built for fast water, moderate water, and slow water, so that no matter what a stream throws at you, you have a box that will work, unless there is no running water or the stream is in a flood stage.
I prefer running the fast water box over a slow water box any day, because the rate of feed on the slow water box has to be done very, very slowly and takes a lot of time to run material. Today all my fast water boxes ( Long Toms ) are set up for river use, or as a high banker, or as a recirculating wash plant box.
To test for water speed and fall on a box, throw in some different size pieces of lead shot (small to medium ) and see where they stop in the box. Next throw in a handful or two of screened material. Most of the material should walk not super run out of the box and the lead shot should go no further down than the second riffle. Adjust the box and repeat until you like what you see, for it is your judgment call on this and it will take some practice to get the hang of it.
Next to test the size, shape, and spacing of the riffle system while the water is running through the box, simply throw a cigarette butt in at the top end and watch what it does. The butt should go over the first riffle and pause, then go over the next riffle and pause, etc, etc, all the way out the box. This mild break in flow is very important in my opinion for dropping gold into the traps.
I have ran thousands upon thousands of 5 gallon buckets of material over the last 40 years. So ask yourself this, how many buckets were ran through the sluice box you bought before it was put on the market and I am not selling boxes here, just putting some things to think about out there, that are not talked about very often or at all.
To go into greater detail on this would take many pages to cover this subject fully from start to finish regarding sluice box style and design. Also it would most likely take me a week or so to write it all up, so this will have to do for now. It would be a lot faster to show and explain than to write about it. Ron