I need a cryptographic hash function that will hopefully be strong even in 50 years. Performance is not an issue. Calculating and verifying can take a huge amount of time if needed. The size of the hash can be huge too (let's say up to 10MB). Collisions are not a big threat. I mainly want it to be second-preimage-resistant.
Of course I have no problem using one or more existing hashes.
My first thought was:
$h'(x) := h(a_1 \parallel x) \parallel h(a_2 \parallel x) \parallel h(a_3 \parallel x) \parallel \dots$
where $a_n$ are known prefixes. But I don't know how good it is. If $h$ is md4, how hard is it to find a second preimage to $h'$?