So in DEMO 5 chests aren't hollow inside, but here is an easy fix:
Download this texture file (right click -> save image; DON'T GIVE IT A NEW NAME!!)
Then you go into your user directory, Mine-Imator, Other, textures (e.g. C:\Users\<yourusername>\Mine-Imator\Other\textures\) and copy the file you just downloaded in there (AGAIN, THE NAME OF THE FILE MUST BE "mobs_size1_frame28"). If Windows asks you if you want to replace the existing file, click "YES".
After you restarted Mine-Imator all chests, existing or new, should be hollow now.