Making and Installing New Centerboard

I would like information on the construction of a new centerboard for my Soverel 48. The boat is #9 of 16 made by Soverel Marine. I cannot find much information about this boat and the company no longer exists. The centerboard trunk is in good shape. There are two holes, one on each side that appear to be the location for the hinge pin. The cable, block, winch and cable tube are also in place.
My questions are:
Should the core material be steel? 3/8”? 1/2”?
Polyester resin and cloth to build up the thickness?
Is counterbalancing or adding weight necessary?
How should cable be attacked and what kind of hardware?
How is the hinge pin install through the trunk to support the centerboard?
TIA

Welcome to the Forum Duvglas.

Here is a potentially useful contact - a gentleman asked a similar question on Cruiser's Forum a couple of years ago, and a forum member called 'pressure drop' responded with a contact who used to work for Soverel.
You could sign up with this Forum, and ask pressuredrop for his friend's contact details - pressuredrop was last seen yesterday on the Forum.
Soverel 48 owners - Cruisers & Sailing Forums https://www.cruisersforum.com/forums/f47/soverel-48-owners-253380.html

What is the inside width of the trunk for the centreboard?
What is on the end of the lifting cable - or is it just that, a cut off end?
If it is a cut off end, is it possible to remove the cable, in order to fit a thimble and a swaged connection? Once you have a thimble, then a shackle could be used to attach it to the centreboard.

Re the hinge pin, is there a hole going the whole way through the keel from one side to the other? I am thinking probably not.
If this is the case, is there access to the top of the centreboard casing from inside the boat?

Is the boat currently out of the water? If so, can you post any photos of the keel, or what you can see from inside?

These show the cable and centerboard trunk. Boat is back in water for now will haul out again back in US. Width is just over 2” if I remember correctly. The holes do not go through from one side to the other and there is no access from inside. The post you referred me to on the Sailors and Cruisers forum was ours when we bot the boat. We did not get response when we sent a PM.

A plate that is 3/8"-1/2" thick would be exceedingly skinny for a boat of the size being discussed and would usually be found in a sub 20 foot daysailer.It might also tend to rattle around if you anchored in any kind of swell.My best guess is that you ought to be aiming to fill at least 70% of the width and how you raise or lower the beast is by means of a sturdy wire,if the image file is reliable.Which I would tend to believe it is and a close look at the photograph up the slot shows what would appear to be an area of un-pigmented glass around a blanked off hole,which makes me suspect that a former owner did some work to bring about this situation.I have a feeling that if you got hold of one of those snake type inspection cameras and investigated the sides of the slot,some clues might be found regarding pivot location.

I can't see any reason anyone would NOT want to seal the hinge pin in, so I'd assume it is buried. Locate the ends and sand the finishes. If under the tank, have fun...

I think it almost certain that the dark area on the side view of the boat is the ballast keel.Which appears to be the location of the pivot,so no leak risk as the pivot is outside the hull.A close look would probably give clues.

On a boat your vintage and size I would expect the centreboard to be solid metal, either steel or bronze. Flat plate with rounded corners, not something with a NACA profile. Thickness as much as the slot permits and still allowing some space to move (1/4" or so each side).
The pin is most certainly inserted straight trough the ballast from one side, and is retained by plates that were screwed over it at each end. You will probably find some evidence of this in the form of a counterbore.
Pennant was probably attached with a shackle, or possibly with a forked terminal swaged over the cable.

My advice is to remove the cable and clean the tube really well (bronze brush), then block up the boat and make a mockup of the centreboard in wood, so you can properly locate the holes. After you proved function, have the centreboard cut from a piece of steel of appropriate thickness and round the edges with a grinder and paint it with a good system. Bush the pivot hole with plastic (ptfe, pom, uhwpe, etc.) and use a high grade of stainless (aquamet, nitronic) or bronze pin. The cable can remain steel or you can switch to dyneema.