I think this is a known problem with this set-up. Haven't done one for a while but I don't think the bolts are a particularly tight fit in the slave cylinder holes. You might be able to swivel it closer to square and retighten. Serrated lock washers might help. With it cocked a degree or two, the natural action of the rod will want to swivel it, and over time it's likely to revert back to where yours is now. I think I would only oval the front hole if I was going down that route. This might interest you.
http://www.sw-em.com/clutch_slave_o-ring_analysis.htm
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