To add liquid to my WSM pan, I open the door & use a really long Christmas tree waterer funnel, not quite like the one in the link that is already half off at home depot ($2.48). I just put the plastic tip in the water pan & rest it on the edge of the water pan as I pour from a pitcher into the cup on the other hand. So far, it hasn't melted the plastic.
https://www.homedepot.com/p/Christmas-Tree-Funnel-HT300-24/203367459I also bought a beer bong at a garage sale for the same purpose, but haven't tried it yet because I thought the flexible spout might not work as well and the fact that the spout is a lot bigger seems more likely to cause problems when filling.