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What is the screw going to be doing? It's not holding it down, because the weight of the post will do that; it's unlikely to stop wobble because the leverage of a post plus panel in the wind will overcome that. I'd go with wooden shims or possibly take a hammer and (using a block of wood), try to bend the sides of the holder in to make it a more snug fit.
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Ta, wooden shims is what @dbr suggested too.
It's not a fence they're for but the outer part of a decking veranda thing on our allotment shed, pictured. Will have clear corrugated roof & is using mostly reclaimed materials.
Securing fence posts upright in spike holders... 7mm too big or just drive it through the metal too? Regular 5mm don't look heavy duty enough for the long term?