Thanks to the pictures that were uploaded, and a random bloke on ebay that was selling an old zodiac and sent me a picture I was able to bodge up a plan. It seems that the design has changed a bit over the years, the ones on my dads old boat were definitely a sort of oval cross section.
The local timberyard doesn't have a lot of hardwoods, I settled on some 1 3/4" square oak, 2m in length. this works out at about 48mm, so it was a little shy of pagick's dimensions (I've just realised that you are in the USA and did the measurements in mm - thanks
this always feels like pulling teeth in work meetings
First thing I did was to put in the slot. I had a 10mm router cutter which wasnt deep enough, or a 12.7mm which was. The boards were about 11mm thick. Ideally I'd have ordered a 11mm bit, but I was eager to get going, so I used the 12.7.
The floor has "feet" on the bottom of it consisting of two trips of wood on each panel which are 23mm high. I decided to put the slot 20mm from the bottom of the stretchers, as I was a bit concerned about stressing the seams right into the corners of the boat.
I fitted some guides to a router and cut the slots in:
Next step was to add the taper. I would have run it through a table saw, but Ive been watching a guy on youtube called "Tips from a shipwright" He insists on planing anything like this, by marking lines along the length and tickling away with an electric plane. So I tried this.
It worked quite well
Even with the pictures, the design is a bit of a stab in the dark, I may have to tickle them up when I do a test fitting, but they look the part. My main concern is stressing the seam by jamming something too large into the gap between the bottom and the tubes. Close up there are a couple of blemishes where I slipped with the plane and router, but from a distance they look ok.