So... I dont get this.

Your dad thinks tubs are "cruel"? Why? Because it's plastic?
Instead he wants you to build a smaller enclosure... thats going to be harder to clean... and harder to keep warm....

Just looking at that from a keeper standpoint, I would NOT want to disinfect that every week, much less wrangle around with the humidity and temps. Since they will be different... And as everyone has stated, its much to small.

Good luck with the chat with Dad.