Thanks Robin,

The G10/FR4 is just basically fiberglass, and I have the filter installed because I don't want everything to get covered in glass dust more than anything else. The marine ply is much cheaper than the fiberglass so if I can get that to work, I will probably go down that route instead.

We tested the glue 1/2 method with a plastic core. My main concern here was that the both the strength and shape of the mast is critical, and if I vacuum glue two long halves together, it's possible to get them out of alignment, but it's also possible to glue them back together with a lengthwise bend in there, and if there are any voids in the glue, side/side delamination could create issues also. Although it's probably more work, that's how I arrived at the solid core outcome. 2 halves may be an option if I can address the above 3 points though

Regarding the pillars and sanding, I am not planning on sanding the piece down after machining it, as I thought pillar removal would be inaccurate and time consuming for me ( both to clamp all the pillars down, and cut them off ) - which is why I thought the female mould would be a better option, and the female mould with vacuum clamping probably the best option.

We thought about using Ali but it is more expensive due to shipping to get out here, and it's completely incompatible next to Carbon fiber in a salt water environment.

Thanks for the suggestions
Nick