Helium filled balloon ..
Front wheel hub Dynamo + Busch & Muller Eyc and Line tech flat taillights wired to the hub dynamo ..
this is probably the lightest Hub dynamo ever made
SON XS
Busch & Müller: LUMOTEC IQ2 Eyc
Busch & Müller: TOPLIGHT Line plus
(or the even smaller
Busch & Müller: TOPLIGHT Mini )
They replace the reflectors on my Brompton and are not much larger than the reflectors they supplanted ..