Not sure if I saw an answer on the forum elsewhere, but I have the same problem. I have a MIG Welder so I thought of cutting it and adding a new plate of metal welded back in place that represents the same part. Just one thing I did come across are these Nuts that have Bearing Surface Sleeve Washer. There is one side of the washer that contacts the drop out and it stays still even when tightening and loosening.
There are other DIY builds done that I saw buy the part and replace it, but it does require Torch Brazing. Not sure where I saw that thread, though someone else may chime in an know. https://framebuildersupply.com/search?type=product&q=dropouts These are parts that would take brazing, but maybe I could use my MIG welder?
This tension adjuster seemed like something I might have used, not a safety device really, just that I welded on parts to make an adjuster when they sell them. Not sure how well any of this stuff is made.
The repair is simple: the reasons the dropout is open is that it's relatively thin mild steel.
It's a cheap coasterbrake bike, just use a small sledgehammer and beat the lower edge of the dropout back to the stock opening width. I would also attempt to track down some hardened, 9/16" steel bicycle axle washers with serrated edges to keep the wheel in place against the pull of the engine. The reason your dropout is opening is because you're not using any washers or cheap washers to start with.
I've done this repair a half dozen times. You always note with the owner that the repair you did is only temporary and they should buy a better bike. Also note that you just work hardened the steel you beat back into shape so it might live to see another few months of use But I would get a new frame or better used frame before it opens again.