a simple cube to show off infills obviously it can be bed size down to 30mm square, smaller than this doesn't really show off infills, i found less than 40mm doesn't show off gyroid that well

make your cube with or without curved corners pick the infill you want then set your slicer to stop/pause at the desired hight, make sure to stop before the top layers start printing

my cubes are 50x50x50 stopped at layer 50 which gives 10mm hight

the file added is for a cube 50x50x50mm smooth corners

just stop the print at your desired hight 

prints in the pictures are

GYROID

CUBIC SUBDEVISION 

CROSS 3D

CUBIC

OCTET

CONCENTRIC

CROSS