I haven't found any plans for a traditional wanigan but I have one myself that I made out of plywood. It looks the part and works well.
The main thing is to ensure that it fits your boat properly - no point in having it rattling about all over the place.
Mine fits in just behind the centre thwart and can be used as a third seat if neccessary.
It's about a metre long by almost half a metre wide, when it fits in the boat it reaches from the bottom of the boat to the centre thwart. I've fitted three flat metal loops that a piece of webbing slides through to fasten it to the thwart and keep it secure (They are actually the metal "keepers" from a garden gate latch assembly) I have other lashing points on it too but those three will keep it secure unless I'm on pretty wild water.
The lid is strapped on with long webbing straps that go completely around the box through more of these metal loops and fasten with fastex buckles at the top. I keep meaning to sort out a more elegant solution though - maybe leather straps screwed to the side of the box.
I made it from 8mm marine ply so it is pretty sturdy but possibly heavier than it needs to be.
I started out with a sheet of 8mm marine ply and some cardboard from a large packing case. The cardboard was so that I could make templates of the curve of the hull so that I could make the box curved to fit it.
The base of the box itself isn't actually curved however, only the two sides. The base is actually flat as is the two ends and the lid.
I constructed the sides of the box by routing out dovetails in the side pieces and the end pieces for the two ends to be joined to the sides and then by routing out slots for mortice and tenon joints to hold the base on.
The lid is close fitting with a groove routed all around and a tongue routed out of the top of the body of the box. The groove in the lid was given a bead of kitchen sealant all around it so that when the lid was strapped down with the tongue in the groove of the lid the box is pretty water tight.
Before putting it all together I routed out a load of vertical grooves inside the body of the box - I made up seperators out of 4mm ply and I can slide them in and out of the box and move them into different positions depending on what I am carrying. I also cut slots into the seperators half way down. Then I made up some other seperators also with slots cut through half way - that way I can shift things about and customise the inside to fit what I want to carry - it works like the cardboard seperators inside a wine case.
As I put it all together I glued it with waterproof wood glue and made a bead over all the seams. th exterior got several coats of yacht varnish but I left the inside unvarnished so my food doesn't taste bad.
The lid has a nylon chopping board inside it so that I can flip it over to use the top of the wanigan as a prep table, then turn it the other way round to use as either a table or a chair. The board is held in with swivel pegs so I can take it out easily to clean it.
Hope this helps
All of the above is my opinion - at least it was when I wrote it. It might have changed by now though 'cos it's not all black and white.