What makes a material not block radio waves?

I'm working on a small transmitter, with a LoRa-module in it. I'm trying to figure out what material the container should consist of, and as the LoRa-module is indside the container, the container can't be blocking the signal. (I may later add a antenna).


Do i go after the lowest electrical conductive material, or is there another parameter i should look for when determining the material?