Think you will you explain that don't really understand. I have never cast in resin so if you could explain please. Thank you for your comments.
I have never cast in resin but there are people who do .... perhaps somebody on IAP would be interested in giving it a try.
I may not understand the process by which you got to your final pen so my suggestion may have no merit.
Your pen has a bulged shape. . I was thinking of filling the cavity with resin.
The resin would occupy the region between the brass tube and the inside surface of the wood barrel.
That might be possible with the wood shaped as it is now.
But, I believe a tube in casting would be easier to do .... IF .... (and it might be impossible, depending on your process of getting to this final shape).
Here is the IF ....
Is there a stage of your process where the wood has a cylindrical outer shape, with the cavity as it is now, and with the brass tube in place as now ?
If the answer is YES, then doing casting should be an easy matter .... it is just a matter of filling the cavity with resin and then shaping to present shape.
I hope I am making my thoughts clear enough .
It was just a thought I had. . The result would be a concealed brass tube plus the openings through the wood filled with an attractive colour resin.