A modular drop riffle sluice with a wide flare (30cm/12in) for better water flow, and a V ribbed section up top so you can see when you're on the gold. Approx 15cm/6in wide riffle section.
I think it would work best with one V ribbed section, two middle sections and one end section...and the flare of course. That would make it 70cm or 28 inches long overall, with 7 drop riffles.
At a minimum I would use three riffle section as show in the attached pictures. Whether or not you use the ribbed section is up to you. If you'd prefer an extra drop riffle then just replace the v ribbed section with a drop riffle section.
Requires minimal support during print.