A better building block?

Its no secret that I love castles and would love to build my own some day.  I have been thinking about the best way to accomplish this and came to realize that quarrying stone for the outer and inner walls would be very labor intensive and very expensive. I then started thinking about creating a castle from cement blocks.  Now I would think your standard cement building block just isn't very attractive and the need for all the mortar in the joints and the fact that it has fairly thin walls limits how high you can stack it as well as how stable it can be.

Now one way to combat this is to include some type of internal rebar structure and fill the block with cement, but that just doesn't seem like the best way to go to me.  I was thinking more along the lines of interlocking cement blocks with hollow cores, but much thicker and sturdier than the blocks currently used.

So I've been doing a bit of sketching and came up with this idea:

As you can see this block is base don a hexagonal design and the protruding "tab" on the top would slip into the bottom of the next row effectively locking them into place.  One thing I liked about this design is the ability to stack these at a 45 degree angle, not just at 90 degrees.  One thing I dislike with the design is the perceived weak spot in the center of the block, so I did another revision:

This block feels much sturdier to me and still allows for stacking on the diagonal if the unneeded corners are cut.  This would produce a very sturdy wall indeed.  I am thinking about this block being 6 inches high x 12 inches deep x 24 inches long and the top tabs 2 inches high.

My next move is going to be to try and create a 3D drawing of both block to see about getting them printed via a 3-D printing service such as shapeways.com in order to play with them and see how well they can stack, etc.  My only issue at this point is learning an acceptable software package in order to create the necessary drawing to get it printed.

Anyone happen to be handy with 3D modeling software?

Any ideas or feedback is definitely welcome!