There is two DIY easy methods to low the reservoir temp.

One is put a fan blowing into the res, and a little aperture in the opposite side. Evaporation low the temp. Take into account that you must refill the res more often and with less nutrients concentration.

The other is a bit more complicated, and its useful when you need to use a humidifier. Use a ultrasonic humidifier, and attach it a duct wich passes through the res. This simple mecanism can low up to 10Âș, depending of the res temp.