When joning of float 2 the bottom wouldn't fit very well. I'm quite sure it's because I left the foam strips a little too long, and hence fitted them with too much tension between the keel flange and the deck flange. I didn't have this problem - at least not this much - with the first float. Probably because the first float wasn't build of the first floathalf I stripped. That half was hanging on the wall all the way from the beginning, and is now in float half no. 2.
I sanded down the entire keel, and glued a new foam strip on. After that I could shape the keel without much trouble.
For shaping the deck edge I scribed a line 2 cm off the edge, and used a jigsaw (angled 45 degrees) to saw a strip of. After that I sanded the radius with a longboard. It was not easy to get it right but I doubt there's a much easier way. Anyway, I won't have to shape a comparable length of edge again, so it's no use for me to wonder if I could have used a better method.
If you want try this method: don't saw too near the line (as I did on a few spots). It's impossible to saw accurately enough and you'll see slight depressions later. Instead use a longboard to sand the jigsawed edge exactly to the scribed line.