The following applies to vehicle systems in a spacecraft, which the book says you can do.
Depending on how much money the players have, some things can fairly easily improve the comfort level.
Bunks are Cr200, take .25 tons and give 1 CP.
An Entertainment system gives 1 CP per CR2000, and there is no limit on how many times it can be improved.
Common areas are 4 CP per ton and costs Cr400 per ton. So see how much common area you have, find a decent machine shop, and install Cr400 worth of vehicular common area bolted over your janky stuff.
A full one ton fresher yanked out of a well maintained vehicle gives 8CP.
Stick a galley in a common area.
Also, instead of imposing a -4 DM, it may be better to calculate the CP of a Swank model of the craft, and then use the % of jankiness against that total. Just remember that some of the space for HG berths is life support, while in vehicles that is calculated separately.