The illustration shows the only way I have been able to stop through-deck chainplates from leaking. The ‘bonnet’ is made of ¼in (6mm) canvas Bakelite, also known as Tufnol or Novasteen.
It has probably gone the way of the dodo, but can be found in old switchboards or similar. It is
an inert material and can be worked with woodworking or metalworking tools.

The secret is to use butyl mastic for the sealant. This is the old windscreen sealant and is a little like Blu-Tack. It stays sticky for years, so it can move with the chainplate while your boat is lying on her side with the sails flogging. It will then re-adhere after the movement stops. Silicone rubber or Sika are not suitable, either under a bonnet or a welded flange, because if stretched too far after they cure the seal will break and they will leak, permanently.

The top of the bonnet fits neatly around the chainplate, with the underside of the hole opened out to leave room for the sealant. If the deck is opened out around the chainplate it will allow a little more sealant, but remember to seal the exposed deck edge.

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