You know the problem: the only kit that covers the model you want in the scale you want is inevitably full hull. So what is the easiest way to convert it to a waterline model suitable for wargaming?
Do not take a saw to the plastic; that way leads to shattering.
Use a sharp knife to score the unmade hull sides along the waterline. Many kids have lines marking the waterline as painting guides... And these work equally well as scoring guides for the blade.
If not then mark the waterline with tape. Airbrush masking tape is fine.
Use the knife lightly, repeating on a small length, say two inches, as many times as required. Resist the urge to press hard and take your time. You are trying to score the plastic, not cut it.
When a section is suitably scored, gently bend the plastic to snap it along the score line. Repeat along the hull half until it is broken in two.
You should have two halves as above .
Cement the two hull halves to the decking and check for rigidity. It should be fine but add supports as needed.
The underside should be pretty square but there will be slight notches and bumps.
Now sand down the model base on a flat surface, gently pushing the model across the sandpaper. Resist the urge to push hard.
The final waterline model should sit neatly on a flat surface.