Congratulations on the construction and installation of your horns. May your pleasure at the results reward your effort and expense. I find their installed appearance even more geometrically striking than anticipated and I suspect now that some of the design choices I wondered about earlier can be attributed to a conscious or instinctive design eye.
As for the horn/drywall joints, basic ideas have been covered but here are some further thoughts:
1 - Settling:
In addition to vibration, remember the house framing is going to settle under the weight of the horns and the horns themselves will bend and warp a little as they respond to their own weight and the temperature and humidity in the attic – (they are not waterproofed on the exterior). This settling is going to expand the gaps over time and you may want to anticipate this.
The idea to keep the horn free of the wallboard/joint compound with a gap covered by some trim or thin fascia (face board) that is free to move with the horn but unattached to the wall probably can work even with serious settling.
2 – TRANSLUCENT silicone fill
For fairly thin gaps you can fill with translucent silicone AFTER painting. The color passes through the translucent material, which should be invisible for this use. You don’t paint over this so the poor paint adhering properties of the otherwise excellent silicone material is not a problem – it’s why they sell this stuff.
3 – Scoring or precracking
Create a deliberate crack and keep the horn and wall separate. You can create a thin straight and virtually unobtrusive gap by filling to the horn and scoring with a razor blade or utility knife and straightedge. If you are fussy, you can do this again after painting and slice through the paint with an even thinner slit. This should eliminate the tendency for unattractive irregular cracking where some cracking is inevitable. Deal with any widening gaps later.
4 – Accented foam stuffing
It you decide to use a fairly wide gap with some foam or other expandable material there is the option of giving this gap a distinctive appearance instead of trying to make it unobtrusive. You can loosely wind a colored ribbon or some interestingly textured material around the foam creating a contrasting border. The right person with good taste could make this work for others it’s an artsy-crafty flourish.