Horizontal dropouts have been around for a century, both forward and rear facing, and rear wheels have stayed in place fine with no heroic measures or the need for added security devices. This is true both with QR and nutted wheels, with and without serrated washers. Super strong riders have climbed the alps, and sprinted on the track without their wheels moving so the first thing you should be asking is Why is my wheel moving?

It could be that axle rotation works the nuts loose, damaged threads or axle length are keeping it from being tight enough even when it feels like it is, the mating surfaces lack adequate bite to the dropout, or the dropout itself lacks flat parallel surfaces making a good mounting impossible.

Don't spend money until you've taken the time to analyze why you cannot do something that millions of people have had no trouble doing for decades. Once you know why it's slipping, and not before, then you can solve the problem. Trying to fix it without knowing the true cause is like trying to patch a tube without finding the puncture first.