Thank you for the replies.
I changed the bottom plate to a 6mm (actually 5.5mm!) plat.I also made it longer, and it looks stronger now. Thanks for the tip.
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I also cut another top plate which will cover the full length of the mount. I think it makes more sense.
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JPS, I totally understand you last point, that makes sense. This was my first idea too, but while drawing, I discover that it would give a "pointy head" to the triangle, and the contact surface with the bottom plate would be to small. But now, with this longer and thicker bottom plate, it would be definitely a better option. But honestly, if you guys think that this version is OK and will be string enough, I'll leave it like that. Just let me know!
Thank you for your inputs.