I can growl (Tim Foust/Avi Kaplan-style), but my low growl notes sound very airy.
Is there any way to get them to sound "fuller"? Or at least minimise the wind-like noise?
To minimise wind noise a few things come to mind.
It could be down to the type of microphone you are using, the frequency response and acoustics.
Generally compression, EQ (away form the high end for the growl possibly), de-essers, noise gates and any typical type of processing would help, particularly for "fullness ".A Focusrite Voicemaster is a good bit of kit to look at which has all the vocal processing things you may consider to beef up the sound.
Hope this helps!