If you've not seen it already, Halo's page about their 15mm-12mm sleeve has some useful information about compatibility with fork dropouts. https://www.halowheels.com/shop/spares/hub-spares/15-to-12mm-adaptor-sleeve/
Also, some Halo hubs allow the endcaps to be swapped. Worth checking if this is the case for your prospective hub.
Bugger! The fork has a lip at 19mm, oh well. Looks like I need a hub with swappable end caps then… …had another look and I think the ends are swappable :)
@Chaley started
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If you've not seen it already, Halo's page about their 15mm-12mm sleeve has some useful information about compatibility with fork dropouts.
https://www.halowheels.com/shop/spares/hub-spares/15-to-12mm-adaptor-sleeve/
Also, some Halo hubs allow the endcaps to be swapped. Worth checking if this is the case for your prospective hub.