Okay i'm new in here so don't blame me for anything.
I was looking arround at this forum as i plan on building a carputer myself one i have a car. I want most of it to be DIY so i was also looking for a DIY PSU (DC-DC inverter). And after a lot off looking arround (not using the SEARCH button ofcourse ) I found this DIY power supply Mastero MK-4.5 250w 12v DC-DC ATX PSU + SDC . Maybe that's the thing you were looking for.
An 230Volt inverter and a normal PSU is not the most battery life friendly option to go as most "normal" PSU are not quite efficient. And then you have the energy losses in the power inverter.