Large amount of heat generated by AC to DC power supply
150°F (65°C) is not an excessive temperature for a power supply. 

You have the supply enclosed in a box where the ambient will continue to increase. As diffidence between the power supply and ambient decreases (ambient increasing) thermal convection to cool the power supply will decrease. 


  It's running at about 120 watts. Can't get much more inefficient that
  an incandescent bulb, and this thing seems to be running hotter than a
  100w bulb.


Your reference to a 100 W light bulb is appropriate.  Put a 100 W light bulb in a closed box and you have an easy bake oven.  A 100 W bulb gets very hot like 4000°K filament.

A fan from the box or mounting the power supply to the enclosure so it can conduct heat away for the supply would be suggested. 

A small, very quiet fan like the $9 12V, 13.5 CFM, 30 dB Sunon HA60151V3-E01U-A99  would likely be sufficient.


I did not see where the ad you reference specified an efficiency. Actual efficiency peaks at a specific capacity like 95%. Actual is generally less. I find many of the vendors on Amazon lie about such things so I would doubt you have 89% efficiency.  

I recommend Mean Well power supplies for pricing, reliability, warranty, efficiency, and distribution.
LINK: Standard LED driver catalog.



UPDATE


  even at 80% efficiency, that would be a 38 watt bulb, not a 100 watt.
  I think that would be far too low to generate this temperature level.


The point is heat is being generated with nowhere for it to go. The PSU heat must have a path to the outside air. 

The current heat transfer path is by natural convection from the PSU to the air inside the box.  Then conduction from the air inside the box through the steel sides of the box.  Then finally natural convection from the outside surface of the box to the outside air. 

The highest thermal resistance is the air between the PSU and the sides of the steel box.

Natural convection cooling between the air in and outside the box requires a difference in temperature to be effective.  The air temperature in the box has to rise to high enough for the difference between the air out side the box is high enough for natural convection cooling to the outside air.  

As the temperature of the air inside the box rises the convective heat transfer from the PSU to the air diminishes and the temperature of the PSU rises. 

The problem is the heat transfer from the inside air to the outside air is too inefficient.  

So either you mount the hot part of the power supply to the box to use conductive heat transfer to the outside of the box and/or improve ventilation which may required forced convection with a fan.