Use a writeable archival CD and/or DVD MAM-A Archival disks and/or add a couple different flash devices (USB, CF, SD, etc) formatted with different file systems. As well as the full datasheet for the flash IC(s) contained inside the devices. If the form factors are non-existant, someone could still read the flash device with some kind of future flash reading apparatus. Or at least carve the full part numbers for the memory sticks and ICs into stone so that the exhumer can look up the datasheet using his/her hive-mind-galaxy-web implants.