if this is for you to actually try to build your own housings (as opposed just being curious) then the cliff's notes from my experiences are:
1. get a VERY solid (preferably metal) template to use. it can be done with a good drill and a good hole saw, but if the template is no good, as Mono4lamar mentioned, you lose precision ... by the bucketfuls! actually, if you're interested, there is another tool that some of the smarter/more experienced guys had recommended, but the name escapes me at the moment. i will try to search for the thread and link it later.
2. put your runner ALL the way through to the chamber, then machine it.
3. Devcon Aluminum putty ...