Here's a link with great photo's of several "styles".
http://www.knifethrowing.info/target.html
I have built several of the type using sawed log pieces,,Ash tree's and/or cotton wood works well even green cut.Willow IS the best for durability and longevity. 30" diameter or better and 10-12" thick is nice and will last for years. If 10" thick, after 2-3 years heavy use, you can chainsaw or face the block to get a new surface.
Hard wood like Oak, Elm, and Maple is NOT desirable!! Sure you can throw hard enough to get a stick, but play dickens to pull the hawk out of the block and CAN break tips off of knifes and the hawk corners.
Soft wood like Pine, Basswood or Poplar work if green cut or freashly dry seasoned but only last a short time.
Of the photo's on the link I perfer/made the tri-pod style heavy stand with thick heavy blocks (like Stefan Roeder's) for annual rendezvous.