We like to have a weather station in the cottage kitchen so that our guests can see the temperature inside and outside of the cottage. I've just changed the batteries on the indoor unit because the display was "fading", but at the same time I discovered the external (outdoor) monitor had failed. It is supposed to be weatherproof but was soggy inside, and despite drying it out the device did not recover.
To prevent the new external sensor from dying in wet weather, I've heat-sealed it in some heavy duty lay flat clear polyethylene tube, together with a 10 gm sachet of silica gel to absorb any moisture. When the batteries eventually die, I'll snip open the tubing to replace the batteries and reseal the existing, or some new, polyethylene tubing.
I had tried to protect the original external unit by housing it in a plastic Tupperware box, but helpful guests repeatedly "investigated" the box in the garden, eventually breaking the catch on the box. The polythene tubing only costs a couple of pence and works well.