YMMV, location also, IE where do you live ?
how is the bike theft? do they care if you bring the bike in with you..?

For me, I got an Armored Cable lock,
A segmented steel tube is over the cable. OnGuard brand, $40.

I packed it in the front Brompton Touring Bag, for a while,
Then took to just hanging it over the handlebars, their M type bar .
So it was always handy..
more impressive looking than it is..

Abus makes a better lock of this type as noted in BikeRadar tests,
Their reviews are destructive testing, best way for a lock.. IMHO,
... try to break it, tool escalation, then noting what was needed to get it open.

Touring bag has plenty of space, they have a stiffener
that the loop can go behind, to prevent wear and potential tear,
on the inside of the fabric , a piece of Coroplast got added .

then the lock if not needed goes in there, shop says 'no bikes in here',
the lock can be deployed.

Any how , I note, the Heavy rider, Bike friday, option, has a front triangle.
that makes locking it up go well, their standard single tube design ,
no similar 'hole'..., secure lock is
more challenging.
the Brompton also offers a triangle in the main section to lock thru..

so what bike you have makes a difference in the convenience of locking it .

Now I have added an Abus link lock in a pouch on the BF, + a chain lock ,
from the same German company..