The only way to reduce the heat is to reduce the power of the LED array. This can be done by decreasing the input voltage or changing the current-limiting resistors on the PCB. Both of these approaches will reduce overall brightness.
The PCBs will get hot by design. Those PCBs are fiberglass so the copper layers on the top & bottom will dissipate heat if exposed to air. The way you have it mounted, though, eliminates airflow from the top (LED) side of the PCB where most of the heat will want to be radiated. Plus it looks like about a third of the bottom layer is covered by your mounting piece. But since there's only a few vias between the top & bottom layers heat transfer between the two is limited anyways. So the bottom layer isn't able dissipate that much heat even if it was fully exposed.
Bottom line: If you want the array to run cooler and last longer the top (LED) side needs to have some means of airflow for natural convection cooling. Maybe some vent holes and raised standoffs between the housing and PCB. Hope this helps.