This summary of the suggested format should have been part of the original mail:
checksum (could be md5 which would be word-sized but likely better to go with sha1 which should provide a good trade-off between cryptographic strength and speed, weighs in at 5 bytes (160 bits), not sure what would be the easiest to verify but could be the whole file with the checksum zero padded, possibly we could hash the header and filesystem separately)