Well, after years of consideration, I finally got around to building a Wannigan.
Many thanks to all the CCR contributors for all the helpful hints.
Here's what I came up with:

I built my Wannigan in the traditional Keewaydin style. I agonized on how big to build it: I wanted plenty of space, but not so big that I'd be tempted to overload it...
I settled on a compromise between the Keewaydin traditional wannigan and the larger Keewaydin "Jewelry" wannigan.
The inside dimensions of my design are 22" x 13" x 15" high.
For construction materials, I used white pine laminated into panels, then planed down to just under 1/2" to keep weight to a minimum.
I also inlaid a piece of Western Red Cedar that I had left over from a previous paddle carving project.
I purchased the Tump from Temagami Canoe Company - one of the few retailers who still sell this type of traditional equipment.
I copied Keewaydin's style of "keying" the lid into the side handles, to prevent movement of the lid in all directions. This design keeps it pretty tight, when strapped down with the tump.

Using a piece of advice from my buddy John at Temagami Canoe Co., I used the table saw to undercut the side handles at 10 degrees, to provide a better grip when lifting the wannigan in and out of the canoe. A minor detail, but makes a huge difference.

I finished the outside with several coats of Minwax tinted Polyurethane, but left the inside natural. I'm currently in the process of building a cutlery tray that will fit inside, resting on two narrow rails attached to the sides.

I built this Wannigan so that it sits perfectly behind the poratge yoke in my Langford Prospector - my favorite canoe. I haven't tried it in any of my other boats, but I suspect it will fit nicely in any full-size canoe.
It is also built so that it will fit perfectly on my winter camping toboggan.

I'm very happy with the outcome.
We're leaving Friday for Temagami for her "Maiden Voyage".
Can't wait.
