i've done this before by first encrypting the file with a temporary key

Why encrypting the file?!
Overwriting the file with random bytes is security-wise the same (or it's even better, since there is no chance that someone could decrypt the file content).
And using random bytes is much, much faster and memory efficient than an encryption.

Update

And overwriting the file with a fixed pattern would be even better! ;-)

Ciao,
Emanuele.