I just built two remote sensors that measure the temperature (thermistor based) and transmits the readings to a base station.
The remote sensor units are based around Nano's and use regular 9V batteries. My estimate is that the battery in ideal conditions should last 6 months to a year. But I am already tinkering on an updated version where I hope a standard 9V battery will last up to 5 years if all works out.
The data is transmitted using 433 MHz modules to a base station (not pictured) that logs the temperature readings onto an SD card. I also have a desk clock built around a Nano, an RTC, and a TM1637 7 segment display that also shows remote temperatures. It shows shows the remote temperatures along with the time in an alternating fashion without the use of any delays as the unit needs to listen for RF signals at all times. In my case I created a timer class and use the timer to decide what should be done (if anything other than listening for incoming RF messages).
As for cases, I used what I could find in our recycling bin... Both of these units are going to be in places out of view so looks are not that important. I am running some tests before I deploy the system. 
