Transom gap filling caulk

[Bassnailer]

Please forgive the title, I couldn't think of a better way to to describe it. While resealing the transom of my boat, I found the putty the manufacturer used between the top of the plywood transom and the bottom of the top cap where it rolls over the transom was crap. There was not enough of this putty to fill the space, it was brittle, and didn't appear to be doing anything. I cut the fiberglass of the top cap off along the top of the transom so I could remove the putty and inspect the the wood. The wood was fine, it was dry and had no rot. I coated all the exposed plywood with system three penetrating epoxy. I then glassed the piece of top cap I removed back on the transom with four layers of woven and resin.

Now, what should I fill the nearly 1/2 inch gap between the top of the plywood and bottom of the top cap with? I have 3m 4200 to seal the back, along with the aluminum plate which goes on top to cover up the unsightly edge between the top cap and transom. I would like to be able to source something locally, and can get fiberglass resin jelly (advertised as waterproof, it's resin thickened with fiberglass dust about the consistency of Bondo), closed cell polyurethane foam, or epoxy and sawdust. From what I can gather, resin thickened with cabosil is what the industry used, but I can't get cabosil locally, and really wasn't impressed by the quality of the putty which was in the boat to begin with. Any suggestions are welcome, thank you.

[CastingCall]

No expert on transom rebuilds, but I bet you've got a ton of time/effort in what you've done thus far (and it looks great!)... so don't skimp out on a less than bullet proof filler now. Get some cabosil from USComposites, or woodflour from BoatBuilderCentral (both from Florida) and mix it with epoxy resin.
Thinking you should be able to mix it to the consistency of caulk, and put it in a plastic bag, cut off a corner and squeeze it in (like a cake decorating deal) to fill the gap. Finish it with a layer of cloth and coat of resin to seal it proper.

[Oldone]

I would lay additional layers of glass until the gap was less than 1/4, and do what 'CastingCall' said.