I am a little concerned mounted a Raspberry Pi in a exterior non-ventilated box. I also happen to live in Texas and it gets really hot here. Last week it was 107 F in the shade on my front porch. With such a high external temperature and the various heat producing items in the external box, I am concerned the PI will be overstressed.
I have a Pi running in an non-airconditioned utility room. The room is usually about 90 degrees on a hot summer day. The core temp of the processor is running about 60 degrees. My Pi is in a plastic box by itself. The processor does not yet have a heatsink installed. While my PI temp is high, it is shielded from sunlight and does not have to deal with the heat of the power supply which is external.
Maximum temp of the CPU is supposed to be less than 80 degrees C. You can check your CPU core temp with the command. You can run this remotely by telnetting into the PI with your WIFI connection.
If you CPU temp is running high, you can add a heatsink. You can also try to find a way to turn off Ethernet, as that Ethernet controller generates a lot of heat. However, if you sprinkler box has direct sun exposure in Texas, then I think you are going to need some other means of cooling the enclosure.