We have an end table with a built-in lamp. The hole in the wooden table where the lamp mounts is much bigger than the bolt that holds it in place. To compensate for this the designer put a large thin washer under the bolt to help hold it down. Over the years the washer has bent & we continue to tighten the bolt to try to keep it upright. Recently we have run out of thread to tighten. Time to replace the washer. I didn't want to re-run the lamp cord through the twisted arm & i didn't want to cut/splice the plug. I needed to make a washer with a slit to pas the cord through but was thick enough to not bend under tightening in this large hole. I made it at Techshop ( techshop.ws ). I took a 1.25" x 1.25" square bar stock of aluminum. I cut mine a 1/4" thick but it depends on how much exposed thread you have with the nut on well. I then put it on the mill & drilled out a hole slightly bigger than the threaded rod.
Then turn the piece on its edge in the vice. Now cut a slot for the lamp cord to go through. For the sake of rigidity, make it as narrow as possible to still fit the cord through. For the sake of safety, file down the edges before passing the cord through. Make sure not to damage the lamp cord while you are installing the cord. Failure to do so could lead to injury or death.