Cool idea for kids to use the walls on creating their worlds , original design from a shapeways store.
2 mins cad: https://www.dropbox.com/s/ou3c9ywy2plsm6x/LEGO_DUPLO_BRICK_2X2.STL?dl=0 Have fun 
Thanks @Rene_Jurack
Da Dice!!!
I have to adjust the STL a bit
STL File updated, link should be the same
Supar

looks good

