I don't know how they'd work on an RC-4 kit, but they are no taller than my Fender bridges. I have one for a P-bass and one for a Jazz. No excavating was necessary ;-)

I have only seen them from internet suppliers. I've gotten them on Alibaba and eBay...right now I am only seeing them on eBay unfortunately. I think they do most of their business with OEM builders, and don't sell much to the public. There are a few shops in the US and in the UK that sell them. Not sure about NZ or Aus.