Andy Thornal Company Custom Waterproof Fly Boxes, from shirt pocket size to boat boxes.  Two sided with clear Lexan© covers make it easy to see your flies.  Construction is very durable.  Positive snap latches let you know that the box is closed, no question.  This box is available is several sizes.

Our Largest Box:

XLarge for the Boat - 2.75H" x11L"x 8.5W -  This big box will carry the biggest flies with ease.  Intended for use in a boat, or kit bag. There are 330 slits in the foam,  Set up is for larger flies and salt water. Slits are 1/4" apart, and approximately 1" long.  Capacity with your biggest flies can vary dramatically, but for comparison is about 50% bigger than the Pack box.  It is also built like a tank, extremely durable clear sides and dual latches for each side.