Deniable data storage

[James A. Donald]

I want to store information deniably.

So there would be a fixed sized block of data, say one megabyte, 
increasing by multiples of 8 as needed.

This would contain various items of information that one could 
extract by supplyin a secret, symmetric, key.   A random key would 
extract a block of gibberish of random length   There would be no 
indication as to how many bits of meaningful data were stored in the 
block, though obviously they would have to add up to less than the 
size of the block.

So one could store one's password list under one key, and the 
location of the dead bodies under another key, and absent that key, 
there would be no evidence that they key, or information hidden under 
that key, existed.

What is a good algorithm for this?