Other options for an air admittance valve besides underneath the sink?
You can (in some less strict locales, like rural areas) put an AAV inside a wall or ceiling. The catch is that you will have to have some way to service it, meaning you need an access panel of some sort.

A lot of cities, however, may frown on an AAV that's not readily accessible. Your mileage may vary.

If I were you, I'd simply leave it as you have it dry fit and get creative with some fittings to make it work. It's the path with the least questions (virtually all jurisdictions allow them under-sink).