You could also use software (see app links below) that monitors folders and automatically handles new files according to a set of rules. So you could have the app monitor your project folder and recognize new bak files being created in any of your project folders, rename or append them (eg. with time and date stamp) and move them to another folder.
One possible setup: 1.) have the app monitor the parent project folder with all subfolders, 2.) set it up to recognize any new bak files created (in any subfolder - so you only need to set it up once), 3.) have it append the files with a time stamp and 4.) move them all to one central backup folder.
That way, all your project backups are in one folder (alphabetically according to the original project name - with time stamp or whatever else you want to add). If you choose to add time stamp, make sure to follow the pattern YEAR-MONTH-DAY-HOUR-MIN, that way all files in your central BAK folder can then be sorted alphabetically according to project name first, then year, then month, then day aso.
At the end, you will have all the bak files from all projects and all different stages of your project all in one folder, plus: any new BAKs created in the initial folder will not be overwritten. You could take this a lot further (backing up the backup folder aso).
One such app is DropIt and it is free (open source / WIN).
I personally also use another free tool from Microsoft (SyncToy) to mirror my entire project folder to another (slower and cheaper) HDD on a weekly basis. That’s an extra level of backup-security (no sync, just A>B mirror).
App Links
Six different WIN Apps for file management: The 7 Best Windows File Organization Apps and File Organization Software
Microsoft SyncToy 2.1 (free): https://www.microsoft.com/en-us/download/details.aspx?id=15155
I am sure, there are MAC equivalents you can google (one is such app is Hazel from Noodlesoft). Just google “noodle” (;