You know, this has bothered me for some time as well. I never thought of twisting it toward a more Secular angle. You may have hit upon a whole new niche.
I'll keep an eye out for something, but all I have every seen are the religious bells you mentioned or the basic brass ones. I think I may just pull out my engraver and carve a big A on it.
My bell is in the style of an armored helmet. It fit with my idea of 'protection'. And to me, 'protecting myself' means paying 100% attention 100% of the time - the bell serves as a reminder to me that vigilance is always a must when riding a motorcycle.
And it has a sentimental value, as being an icon of biker lore - but zero superstitious / metaphysical meaning.
If a bell were available with the atheist A, or even the Darwin fish, I'd buy one!