OP wrote:
"I really need to be able to access older backups"
If all you need to do is "get to your own files" on an older cloned drive with an older version of the OS, just plug in the drive, let it mount on the desktop, and "go after" the files that you need.
You don't have to "boot from it".
That's the beauty of a "cloned drive" -- it will "mount on the desktop" just as any other drive does.
BE AWARE:
There CAN be an issue with permissions when you mount an older drive (that had older user accounts) on a newer Mac.
This is EASILY overcome by doing this:
1. Mount the drive icon on the desktop. DO NOT "open it" yet.
2. Click ONE TIME on the drive icon to select it
3. Type "command-i" (eye) to invoke the "get info" box on the drive
4. At the bottom of get info, click the lock icon and enter your administrative password
5. In "sharing and permissions", put a check into "ignore ownership on this volume"
6. Close get info, and you're done.
You can now copy files from the old drive, and they will automatically "come under the ownership" of your NEW account on your newer Mac.