A Stackable Icecube Tray.
After printing several, stack them on top of each other and start filling the first one. Once it is filled the fluid will automatically flow through the overflow drain and fill the next one. Make sure tho you rotate every next level by 90 or 180 degrees. Otherwise the drains will line up and the water will just drop down to the floor.

Printed using about 38m of PLA filament at 100% infill. I choose 100% to make it watertight.

Designed in Catia V5

After letting some ice freeze over night, I had some trouble getting it out of the shape. I think the imperfect surface finish from the 3d printer gives the ice to much surface area to stick to. A way to optimize this might be to sand of the walls to a smooth finish.
Please post your experience below in the comments.

---Update---
Added a new Version 2. It does not use rounded edges at the bottom of the tray and has a thinner bottom layer. This way I could reduce the filament to 29m.