easy... theres a 0x1040 Bytes sized Header starting with sumthing like "123-dex"( been a while ), theres description and additional stuff for the notes. I dunt have a clue about the format of the header, but to get a working Dex-Drive state you just have to make sure theres that "123-dex" at the beginning, everything else is optional.
The rest beginning with 0x1040 is just a plain 1:1 copy of a Mempak.
EDIT: the definition-string is "123-456-STD" starting at 0x00.