Ducky with that low water flow you might want to get a small tarp from the 99¢ store and lay it over your small dam/waterway to essentially block all water flow around the sluice and direct it all into the sluice. So build your dam and create the spot for your sluice. Lay the tarp across the dam lengthwise, bring the tarp edge over the top of the dam about a foot or so just past the crest or high point of the dam area you plan to put your sluice. You will want to build up the dam height so the water will not fill up and flow over the top of the tarp. You will want 2-3 ft of tarp to extend toward the upstream side of the damn, lay it flat on the bottom/sides and place big rocks around the edges and scatter larger gravel across the top of it to hold it down to keep water from flowing under or around it. Position your sluice on top of the tarp into the opening in the dam that you created for it. You will want the beginning of or widest part of the flare at the opening point of the dam where all water is being directed into it. If you have some bubble wrap, that you can roll up into a 4" wide piece that the length will fit under the sluice and up the sides, you can place that on the damn between the rocks and the tarp so that the tarp is well formed to the sides of the sluice making a good tight seal.
You can Skype me if you have questions about this. [emoji106]