yep good old:
cat /dev/random > file.bin
leave that running for a few days while you go off and do other things (it can only random while you have entropy) and soon you will have a huge random file. if it’s too big cut some off.
but i wouldn’t bother lying and saying it’s garbage, i would say i keep garbage files and leave it at that…they have to prove whether the file is encrypted or garbage.
course there is always the analog hack:
