Acrylic glued to painted alloy panel

I'm attempting to repair a caravan alloy side panel. The usual trick of using filler, smoothing and painting cannot be used due to a difficult to match ribbed pattern on the existing area of panel - so the plan is to stick a flat panel over it, a little way proud of the surface, rather than just the minimum thickness, with neatly rounded corners to make it look a bit more intentional than a patch. I did plan to use a piece of alloy, which might be difficult to get in place flat. Someone has suggested 6mm acrylic sheet might be easier to get flat.
I'm just wondering if I might be struggling first to fix it - if the surfaces were roughened, would ordinary sealant stick it in place firmly and might be struggling to spray paint it?
Harry (M1BYT) (L)
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