I have the pouch that holds the Abus folding link lock
on the water bottle cage mounting bolts, of my Bike Friday.

they have a second, low behind the seatpost cage mount,
I use that site to place the bottle and under the cage I use
some mudguard struts wrapped around the bottle cage
to provide the 2 places to mount the dynamo powered
taillight, that would normally be fitted to a rear rack..

i use a similar light on my other bike that does
have a [Tubus] rear rack..