Check the specifications of your transformer.
Most are rated to supply 100% load with a 40C rise over ambient temp. Calculate your load requirements, they should be no more then 80% of FLA of the transformer.
If by chance you have other electronics inside this same "box" with the transformer, you will most likely want to use a fan. ICs typically start dropping out around 150F. Manufactures specifications rarely state recommended operating temps above 100F