Hey Orin, looking good.
I know you can get marine grade fir plywood in Winnipeg at Windsor Plywood. They were the only place that I could find that carries it without being a special order. It comes good two sides, so that helps. The only bad thing is the price, $59.99@ sheet. It will take six sheets to do your narrow.
Also, good idea with using the rags to heat up the oak to bend. Your idea never crossed my mind as I used a pice of PVC pipe and an electric kettle (something I see on a "google" search). That's what's nice about this NT site; being able to see other peoples ideas to reach the same end.
Keep up the good work, I hear winters coming. ha ha