We have mechanics buildings that use butyl all the time for glazing applications. It's a great material. I'm just leary about it in an "uncontained" situation. It might work.

I just know that silicone will work there.

4200 will work too. 4000 will also (it's not really an adhesive, though it has some of those qualities) -- it's a sealant. The anti-yellowing in UV is good, but in this case shouldn't be an issue? If you're concerned with that, then butyl is black(ish), so....