PCB Mounting solutions in outdoor enclosure?
Nothing beats proper screws to secure a PCB within an enclosure. I do not recommend adhesive standoffs, especially if your board has 120VAC on it. The adhesive can break down with time or temperature, and if the board falls into the enclosure it can either short out or cause the enclosure to become live. This is obviously very dangerous.
My recommendation would be to create a &quot;backplate&quot; (using ~16 gauge steel) that screws into the existing mounting holes in the enclosure. Then add holes that line up with mounting holes on your PCB and screw the PCB down to the backplate using, for example, 1/4&quot; diameter standoffs. I have found 6-32 screws/standoffs to be great for mounting most PCBs.
If your PCB doesn't have mounting holes, then the first thing I would recommend is design a new version that has them. That is by far the best option. If for some reason you can't respin the board though, and you must use adhesive standoffs, use some sort of strap (Velcro?) to go over/around the PCB to make absolute certain that even if it becomes detached from the backplate, it still can't fall against the wall of the enclosure. Redundant safety features are a must.
We may be able to provide more specific solutions to your problem if you post an image of your PCB, but based on the information provided in your initial post this is my recommendation.