What sluice depends on the water flow, volume, and grade (drop). A 30'' with a wide fluke is most versatile.
Best preparation for any situation is to have a small light weight pump so no matter what the conditions you have enough water flow across the riffles to actually wash the tailings out of your sluice. If you have the pump, even a homemade sluice will do just fine. BUT, if you build a wood sluice, it will get heavy when it absorbs water, so paint or shellac it thoroughly. We have flat water here and to run any large amount of sand/gravel there isn't enough drop to generate the water velocity to clean the riffles with any speed. What you see on the gold fever show is not real world. Without water velocity (drop) there isn't enough water through the rifflles to wash out the gravel and sand, so the water just goes around the fluke and your classified material just sits there or takes forever as in slower than panning. Unless everywhere you go there is a pretty good drop that provides fast enough water through the riffles, you will do better buying a cheap small gas powered pump and building a sluice out of whatever is handy, a good sluice can be home built by anybody, it's not rocket science. Otherwise you have to carry the gravel to your sluice instead of moving your sluice to the diggin'. I use a Keene P 100 2 stroke and an aluminum long tom which is very lite,maybe 20 lbs. (power sluice)
But even ahead of all of this, are you at the point where when you 'sample' a creek, are you getting enough color in your pan to know you can (for sure) do a whole lot better with a sluice?
Cold water from cold Iowa, Good Luck with it, Jim