Complex plaster molds for ceramics
It looks like you could blow up a ballon to get the shape of the interior.  Then glue hex shaped objects to the surface of the ballon in the pattern of the holes.  Then build a wire framework over the tops of the hex shapes to hold them in position and to support your mold's outer shell.  Finally apply that outer shell as a series of glued vinyl sheets or paper mache.  Once that outer shell is dry, you can reinforce it with clay or silicon. 

You should then be ready to pour in the liquid clay.

This will be a sacrificial mold, so you will mostly need to destroy it to get your casting out.  To de-mold, remove the reinforcing material and paper mache, then the wires.  Then pop the balloon and push out the hex shapes.  

The exterior of your casting may have some lines from both the wire framework and any seams in the vinyl/paper mache.  It might also have bits of vinyl or paper mache stuck to it unless you washed the interior of the mold with lubricant before adding the clay.  Those problems should all be pretty easy to fix with a little sand paper.