On Fri, 09 Oct 2009 15:48:05 -0400, wrote:

How would you go about forming these slots? What sort of tool?

inside corners. The other alternative if you can get the use of one,
is a mortiser.

Reread your post a little closer and obviously a router bit isn't
going to be much use with 6" posts. The only option I can think of at
this moment is a mortiser and you might have to cut from both sides to
get a through hole. It's going to be a lot of tedious repetition any
way you look at it.