I have duct tape over the opening on my Beneteau 373, which I change once per year (I'm on year 2) due to UV degradation... not a very pretty solution, but it was more UGLY when I had to "evict" some swallows who decided it was a great place to start a family... I'll spare you the details. On my to-do list right now is to find a more elegant solution, and I did stumble across the device at the very bottom of this page:
https://www.rigrite.com/Spars/Sparcraft_Spars/FB_Booms/FB220_boom.php (around $40 plus tax and shipping) which I think would work, but 3D printing a more decorative solution came to my mind as well. Is yours held in place by friction? What does the back of it look like? Are there flexing "clips" that hold it in place? I'd love a custom 3D printed one, but presumably would need extremely accurate measuring to ensure that it stayed in place... The opening on the boom is different on mine... more of a trapezoid with rounded corners... Its gotta be a pretty exact fit, right? How did you go about doing that? I want to make one... I have several friends with 3D printers... but don't know how to describe the measurements to them, and how to get it to stay in place without adhesive through rough seas, gybing, etc. I was thinking about using a couple of diagonal "set screws" to help lock it in place, but have so far decided 6 inches of duct tape was simpler until I come up with a more perfect solution. Advice?