At first look I didn't realise that it was so small, so thats a terrific job, both building and painting.
I did read on another forum that one solution for displacement and stability was to use NiMH AA cells in a waterproof tube mounted under the hull. Doing this added stability, but, since the tube/cell arrangement weighed very little more than the water it displaced, it didn't move the waterline up.
Two torpedo styled tubes, side by side, could forrm a stand.........
I don't think that the length/narrowness of the hull is a factor, rather, the height of the weight above the waterline. Sure a very wide boat will appear more stable because it needs to tip further before the centre of gravity gets past the point where the buoyancy on that side can resist it, but a narrow hull can be just as stable with the weight lower.
Model liners tend to have an awful lot of plastic a long way above the waterline - I remember the Airfix Canberra had so much that it couldn't be made to float anywhere below deck level, let alone upright without assistance.