When I said above that I used paper cardboard from pizza boxes (that so far has not gotten too wet) that I put it in along the front, bottom and back of the bag, so the cardboard is more of a U shape if you look at the bag from the side. And I mentioned that I used a pizza cutter wheel to crease the cardboard so that I could fold it easily where I wanted to. Thus plywood would not work the way I do that because that does not bend very well. But I can see where if I wanted to only have a flat sheet in the bottom that thin plywood would work pretty well.

Note to Staehpj1, one more reason I liked the Carradice is that I use Ortlieb Classic Roller panniers. If I was using Arkels or some other brand of pannier with lots of pockets, I probably would instead use a dry bag like you suggested. But I always want to pack so that my panniers stay closed all day long because they are even less convenient to open up. Sometimes I strap rain gear on top of a front pannier, but other than that the saddle bag and handlebar bag are the only things that I want to open up during the day. The photo I have above in post number 5 shows mostly lunch fixings, plus a 1.5 liter bottle of wine in the saddle bag.