What can I put at the bottom of my cupboards to stop them getting wet?

I have a problem with a couple of the cupboards in my kitchen where the bottom surface appears to get wet, particularly at the back. This is a rented house, (as I am a student) and isn’t well built – the back of the cupboard faces the outside and our insulation sucks to be frank.

Obviously I don’t want it to get wet as I want to keep cardboard boxes of food like rice.

What can I put on the bottom of the cupboard to stop it getting damp? I don’t want to have to keep cleaning the bottom every few weeks. Was thinking of a towel, but thought that it might be a bad idea with absorbing lots of moisture.