Abus Bordo Link locks are like a folding U lock that goes in a pouch
you can strap on,
or as I use on my Bike Friday by their top of the top tube bottle bosses.

mostly I used an Armored Cable lock, OnGuard, though there Abus offers one too,
better reviewed, In Bike Radar's destructive testers opinions.
The Lock in that case, just hangs over the Brompton's handlebars.

FWIW I've seen a Seat post mount use such that the U lock lays parallel
to the main tube.