I agree that the "V" mat isn't the best choice. Granted you can get by with less water flow but it most likely won't grab all of the gold. higher water flows will just blow gold right out of it.
What do you want to make the box out of? Metal? Wood? I've used both and both work very well. The wood is a bit more work to take care of though. How much flow you need is going to depend on the width of the box, they type of materials you're running (1/4" sandy takes less than 1/4" with a high clay content) and what type of surface and riffles you're using.
My current sluices range from 6 ft long X 6 inches wide to 4 ft long X 12 inches wide. I always seem to catch most of the gold within the first foot or so.