I developed this cross model specifically for sand casting: All the sides are slightly tapered so the mold will be easy to remove from the sand.
About 20 minutes modeling (in Maya), 2 hours printing, and 45 minutes casting (10 minutes making the mold, 15 minutes melting the metal, 20 minutes cleanup & paint).
In the attached images, the gray version is the print, and the silver/black is the aluminum cast.
As printed/cast, it's about 7" long.

Printed on Replicator (1) :

• 220 deg, gray PLA.
• HPB off, glass build plate, blue painters tape.
• 2 shells, 20 micron layer height, 10% infill.
  Printed in just under 2 hours.