Exactly, I don't use Steam for it. I use Steam to set up the first batch of files, customise where needed(say add in steam's .dll files) rar the files, upload it to an ftp server.
Next time server is created, it goes to download the files from my ftp/storage server.
Step further possible too, you can set it so the rar files are saved on the remote.
It checks the storage server to see if the timestamp on storage server is newer than remote. If yes, it downloads new one! If not, it uses the rar on the remote.(Can also use it without storing on remote!)
I tried finding the guide on the wiki but failed, pretty sure it was on there though..