Really depends on how much you have to invest in one. I've thought about constructing a portable dredge/sluice box but it does take a certain amount of money to do it. I'd think working with aluminum would be best(the lighter the better), but I'd try to incorporate at least 2 hinges into the design for the box. Then you have to think about the ripples and mat to catch the gold. Lets not forget about the frame/stand, and then finally you'd have to decide on if a pump and hoses will be benificial. Now, keep in mind that unless you have a mule you have to pack this all into the site yourself unless you can find somewhere that you can actually drive to. I take that back, you'd have to have a mule to pack out any large amounts of ore you might find. And I wouldn't put it past the need as I remember seeing a post not more than 2 months ago about someone finding several sizeable chunks of silver.
