Hi, I made myself a new centreboard when my plywood one broke. I used what is sold as mahogany at timber merchants. I was luck enough to get pieces with the grain perpendicular to the face of the board, but made it up out of 3″ planks anyway as that was what I’d ordered. I got the planks ready-planed to the correct thickness. If the grain isn’t perpendicular, i.e. the planks aren’t quarter sawn, you will have to make sure that any tendency to warp in one plank is resisted by the next. I bought an official template to get the shape, though ISTR quite a bit of measuring to get the pivot hole in the right place to get the best allowable angle when the board was fully down. I glued it up and sanded it with a belt sander and varnished it with epoxy varnish. Not sure about the weight, but I reckon that my W’s original board must have been mahogany, 50 years ago.
To prevent the board rattling in the casing, I glued two thin strips either side of the board near the top.