Some model boat fittings can be bought and some are best scratchbuilt. Some probably would be bought but if you forget when placing an order, you end up scratchbuilding them.
If there are any model boat shops out there that carry a full range of fittings, they are few and far between. The nearest one to me is in Leicester, which is too far for a quick visit.
That's why I had to build the lifeboat box for the front of the my Pilot Boat the hard way.
To be fair, it's not that hard. The box is made form 2mm thick plasticard. Cut the funny shaped ends and then a long strip of plastic the length of the desired item. Slice this up to run around the ends. Size is 45mm long, 20mm high and 35mm wide. That looks about right on the model.
I did drill some holes in the bottom to let solvent fumes out.
The basic box was left to harden overnight before being filed to neaten up the joins. Then a strip of the same plastic 3mm wide runs around the edge and some more over the top and bottom. I think these are the joins between top and bottom along with the retaining straps. To be honest, I was modelling a model at this point. Bad. Bad. Bad.
Still, it looks OK to me and my version is lighter than the resin version I'd probably have bought so that's a good thing.