sv_downloadurl is how you set it to download from a seperate site rather than the game server. After setting up your sv_downloadurl, and putting all the files on the sv_downloadurl server you then tell it what files to download using ulx addForcedDownload. If ulx addForcedDownload won't work for you, you can look into resource.AddFile() (use the wiki) or add the files to <mapname>.res files.