Let's look into recommended practices when mining with GPUs. Although mining Bitcoin is no longer profitable with GPUs, mining other coins may bring in some profits. If you are going to use your GPUs for mining, you might as well observe practices that are going to make it easier and safer for you.
First, make sure that your GPUs have adequate cooling and the GPU fans are at least two inches away from any other object. If your GPU is inside a desktop case, it is better to leave the case open. If you are building a mining rig with multiple GPUs, it is a good idea to add external fans to increase the airflow. It is also wise to leave the GPUs exposed (as in not enclosed), so that there is better air circulation. Remember that hot air rises, so it is best not to have anything above your mining rig. As you may have seen, people do use "milk crates" as enclosures for their mining rigs; it is a good starting point. However, now there are professional manufacturers...