I do not know the dimensions of your sluice or buckets but you might want to check out the prospecting section at a Bed Bath & Beyond store. They call it storage containers/drawer dividers/shelf racks but they are all expanded metal and come in a variety of shapes and sizes.
I'm assuming you will classify on land and then take the small stuff out to your sluice in the river. If so then some hardware cloth well stapled to a couple of boards will likely do the trick as classifying does not need to be fancy to work. Of course that is dry classifying. If you want to really add some work to classifying, and its is a lot of work in itself, you can purchase a tapered wire mesh waste basket they usually have a solid sheetmetal bottom that will need some 1" holes in it and then some 1/4" hardware cloth mounted over the holes. The mesh wastebasket I modified for this purpose just slides into the typical 5 gallon bucket. Do NOT fill the wastebasket more than 3/8 full or else you will really have a sore back after the first day of use. Using sturdy gloves pick the wastebasket up and slush it up and down in the water and give it a couple of back and forth twists and the stuff still in the basket will be fairly clean. Now dump the cleaned material onto the ground and detect it for nuggets as most pickers and larger pieces will be in the dump pile IF there's any there in the first place.
Happy Hunting...........63bkpkr
