A simple case for a (Wemos) D1 Mini + BME280/BMP280 + Battery Holder / Battery Pack.

Theres a cutout for a rocker switch which is highly recommended (when using batteries) as you dont want the batteries connected when using the usb port of the D1 Mini or skip the switch and batteries alltogether and just use the usb port to power the D1 Mini. 

The Box can be secured using 2 screws directly or using the (optional) back plate which can be secured e.g. with double-sided tape to any flat surface. The box mounts onto the plate and can easily be removed for a battery swap or flashing.

With deep-sleep configured to wake up every 15 minutes, the 2 AA Batteries will last around 3-6 months, depending on the WiFi signal strength.

Unfortunately I don't have a wiring diagram at the moment, but it's not too complicated and can be seen from the picture.