Your sluice needs to be "tuned" to your particular situation. Water speed, gold size and type of material being run have everything to do with how you should tune your sluice.
For instance: lets say the paydirt is mostly sand and the gold is mostly fine, then the v mat or carpet under the expanded metal or riffle tray would be a good choice.
But if the dirt is coarse with lots of gravels and the gold is coarse and consists of flakes and fines, then you want to run the riffle tray over the miners moss with the option of either v-mat or carpet underneath the moss.
It depends on each particular situation as to what is the best way to set up your sluice.
For example: Water speed needs to be faster when running larger material over riffles and much slower when running fine material over just v-mat. Classification and sluice angle are important details to consider when fine tuning.
Carpet, v-mat, and miners moss are all capture mediums.
The riffles are there only to create low pressure zones and to help break up and stratify the material but mostly to create low pressure zones to aid in gold falling out of suspension. The moss, carpet or mat are there to capture the gold and prevent it from re-entering the flow.
Miners moss is better at capturing flakes and nuggets and not so much on fines, thats why carpet or v-mat is needed under the moss to capture the fines and to also help prevent migration of the fines under the moss and out the end of the sluice.
There are many mats on the market that are far superior to and would eliminate the use of carpet or miners moss as a capture medium but are way more costly.
Here is a mat however, that seems to me to be the best as an all around mat with multiple capture zones that is NOT expensive and would eliminate the need for a riffle tray and/or expanded metal. Only $20
https://www.seriousdetecting.com/product/trimmable-riffled-tpr-light-weight-sluice-box-matting-27-x-10/
Just trim it to fit your sluice.
View attachment 1913434
Hope this helps.
Go for the Gold
GG~