Take a 20x20x20 area out of the middle of your death cube and fill it with hull. Or add 2 or 3 extra layers of hull around the side of your death cube. I instantly ruined your idea in less than 3 seconds unless you have a workaround.
Great thoughts! It's needed to work out details. Thank you.
1. A hole in the middle of a deathcube is not visible, but at least makes it more interesting, even if not externally and forces it to have 8000 less blocks inside to use. That makes a deathcube less viable as a design choice.
2. Since my idea was about surface squares, not just outer hull, the thickness of the hull wouldn't matter. Just the outermost layer. So adding 2 or 3 layers of hull won't matter. By using a bigger hull, you have more surface area, and by having thicker hull, there's less room for hot power sources inside the hull. However, you can still build hull wings/fins for more surface area.
3. The idea isn't ruined. Just detailed more. We definitely don't want suggestions that make the game less fun or suggest features that can be grossly misused, so by all means, please continue finding chinks in the armor of my ideas so I can either figure out how it can be better, or abandon it. (I have been convinced to change my mind before, and it'll happen again.)