After seeing Cunicode's brilliant brick: http://www.thingiverse.com/thing:28240 Which was inspired by http://www.thingiverse.com/uck

I figured my sons Duplos need something similar :-)

WARNING: Children are dangerous! Erhm, I mean: DO NOT LET KIDS PLAY WITH THIS UNATTENDED! Parts could break off, they might swallow or choke on it, etc. Allow use only UNDER CLOSE SUPERVISION.

UPDATE: Added a revised file with tighter fit on the bottom.

UPDATE: The bottom fit seems a bit loose when printed on an up, but the top and sides are all nice snap fits! I have added a revised version which should provide a tighter fit on the bottom.

I quickly dimensioned this based on a few measurement on the original large bricks. Construction is more or less true to the original, but modified slightly in the hope that a home-printed version will be sturdy enough and not de-laminate. Only practice will tell if it will actually fit and hold up to a toddler. Please read the warning and think before you give something like this to a toddler!