I've replaced my rear fender with a Blue Collar Bobbers fender and I don't want to mount the rectifier to the side of the battery box like they recommend. I see that a lot of people have put the rectifier in the airbox.
My question is, does it need to be bolted/screwed in place? Or is it ok to just have it rattling around in there?

The original steel fender acts like a heat sink.  The rectifier dissipates heat not only from the air that blows over the fins - but also by bleeding heat off to the fender.

It is best to have it mounted where it gets airflow - but also where it can bleed heat off to something metal.