minify & disk: fix file permissions

Hi. w3-total-cache is working flawlessly, except that it writes minification cache files using 0600 mask. I’m running a VPS (no cpanel/suexec/etc), and webserver user www-data does not have access to the cached files, which are owned by some other user.
I must change w3tc to chmod cached files to 0664, which on my server is a safe permission. How can I do that?