You can use the free tool Google SketchUp to create models for Twinity.
Here is a simple step by step tutorial how you can get models from SketchUp into Twinity

First step: Create a box, make sure it is centered in the origin.

Second step: Export as 3d model (kmz file extension

Third step: Rename the kmz file into a zip file (by changing the file extension)

Forth step: Unzip the zip file ("Extract all...")

Fifth step: Open Twinity, select place item and navigate to the extracted folder. Open the dae file in the "models" sub folder

Sixth step: Place the item and enjoy your new box

• Download the example kmz file