I just grabbed this from Bunnings.
It's 5 inches in diameter. With a 5ft base in the centre it could be used to help folks visualise a 10ft radius.
Staying within the 10ft range of a paladin's aura of courage could help evade the pant shitting impact of a dragon's frightful presence ability...
.... and set things up nicely for a breath attack.
Thus is the sound of da town guard!