I know what you mean about just making it up, its so much easier when its there in front of you, than sitting at a computer with CAD. Have you seen the design guide for the blocks? looks like you will have to be careful with the spacing as any racking will cause them to bind.

Since they are 4 way loading have you considered putting the x axis rails on their side, that way you just need a flat plate to bolt the carrages to the y axis rail.

Where do you get the ali sections from?