How can I add "Levels" to the Opportunity object?

I want to set up Levels for my nonprofit, but I want it to be calculated from the Opportunity (rather than Contact/Account).

Example: Rather than assigning a /contact/ a level (Bronze if total giving equals $150 - which could be 3x $50 OR 1x $150 gifts), I want to assign it to the /opportunity/ (Opportunity amount => $150 = bronze level).

I've watched the video & read the NPSP documentation on setting up Levels - which says you CAN use levels on objects other than just the Contact or Account object, but it doesn't say HOW. I added a custom look-up field to the Opportunity object relating it to the Level object. But when I try to create a new level, the drop-down "Target Object" only gives me two choices: Contact or Account.

So how do I go about adding Opportunity as a target object option when creating levels?