Attaching to the floor of the boat should be easy. Probably a flange all the way around the inside or outside of the bottom of the console to screw through would work. Round corners, is that what you meant? I suppose if you are wanting to build it from wood and then glass over the whole thing then some large quarter round for the corners would work. You could make quite a complex profile if you had the tools, recessed to accept the plywood panels etc. You could also build it like a square edged box with 2"x2" inside corner braces and round the corners off with a router, belt sander or something similar and glass over that.
If you want to get away from a boxy shape and have something more rounded and smooth then you might want to research making a mold and laying up the solid fiberglass part up inside of it. You could add wood or composite members where needed (helm) as the layup proceeds.
One last idea to look into is the method speaker enclosure builders use of pulling a stretchy fabric over a form that makes up the basic critical dimensions of the enclosure. In your case the helm station, the mounting foot and some removeable temporary framework. The streched fabric forms a more organic shape than a plywood box. Resin and chopped glass is brushed over the whole thing and then a layup is built up over that. Once full thickness is achieved and cured the temporary framework can be removed as the fiberglass console would be a single standalone part. This would be tough to lay glass cloth over smoothly though.
...on 2nd thought it might be easier to buy one...
