Rollcast is on the right track!
Hub Centric Rings should get you some good search results.
They are hub-size to wheel-opening-size specific so do some googling or measuring.
These likely came with your wheels since that hub opening looks much bigger than our factory axles.
Without these hub-centering rings, it relies on the lugs to center the wheel.
There's usually too much tolerance built into the lug holes on the wheels for this to work, causing your wheel to be off center.
If your wheels aren't lug centric and don't have them on the other 4, I suggest getting them.
For the spare though, you could rely on the hub centric ring which is made of plastic, or you could look into having someone fab some extensions or even a circular plate (think barbell weight) to better distribute the weight.
Hope that helps!