Here's what I've done for years to improve clearance on track forks. The same method can easily be applied to a vertical rear dropout.
Fill the top of the dropout with 1-3mm of filler, carefully contoured to match the curvature of the axle, like the dropout was before modification. Note it's absolutely critical that the thickness is matched fairly precisely, so that the stays vertical and square.
I use steel filler to handle the load so the filler doesn't wear. For example to gain 2mm, you can cut and curve small pieces of 2mm spoke to match the dropout, then build around that with body filler.
My personal bike with a fork modified this way has been on the road for 40 years and there's no noticeable change in any respect.
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