I looked through many of the CX articles and it doesn't actually say what the hull is made out of. However, since this particular design was a military transport I had assumed it was made out of a medium-or-better alloy as most warships have. I'll have to ask Uso what these ships are supposed to be clad with, since I think he designed the C3F.
The 'Spacers rely solely on their hull for protection (their ECM array disables their ship shields, remember), and so design ships with a very sophisticated and rugged damage control as a result. Since this model has a modified internal structure I thought it was appropriate to boost the hull HP from 30 (The original HP value of the C3F hull its based on) to 40.
The DR system isn't really designed to accommodate changes other than armor and shielding, so I wasn't sure how to handle it. I thought the best way was increasing hull HP rather than armor, but I can always revert it to normal values if you want.