For folks outside of warzones with plenty of time and access to international shipping (i.e. not OP), there are off-the-shelf adapters that accept DC input and

For folks outside of warzones with plenty of time and access to international shipping (i.e. not OP), there are off-the-shelf adapters that accept DC input and produce the ATX voltage rails with the proper motherboard plug. Search for "DC-ATX" or "PicoPSU", they're pretty neat devices using DC-to-DC converters.