0.9mm aluminium strip should be easy enough to roll neatly by hand round a seat post as a former. Pull it round a slightly smaller seat tube and it'll probably spring back to the size you need.
E.g. https://www.ebay.co.uk/itm/111912004219
That stuff looks like it's guillotined sheet so it'll be even softer than rolled section.
@Jonny69 started
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0.9mm aluminium strip should be easy enough to roll neatly by hand round a seat post as a former. Pull it round a slightly smaller seat tube and it'll probably spring back to the size you need.
E.g. https://www.ebay.co.uk/itm/111912004219
That stuff looks like it's guillotined sheet so it'll be even softer than rolled section.