Very few full size vessels have external keels, usually they are built into the double bottom structure. The bottom of the hull is flat. However potentially unstable models such as paddle steamers may have fixed or detachable sailing keels to improve their stability.
One option if you are uncertain is to build in a couple of fixing points to the bottom of the hull such as brass threaded sockets which can be used to accept a sailing keel, or indeed a variety if you are experimenting! They can be removed when the model is on display or in storage.
Colin
