I got a
B&M Toplight flat permanent for the rack on my folder.
It's not as bright as my other rear lights, (Smart 0.5 watts and a dynamo driven B&M on my Globe).
I know there are breackets available for e.g. the Smart lights to mount to the rack fitting, I didn't want anything that would stick out too farand hence might easily get knocked when stowing the bike on the train.
The toplight s better in this regard. It also incorporates a pair of good reflectors.
Note that although the description says "Extremely flat casing (approx. 117 x 50 x 20 mm)", it isn't actually that flat. the only way you can measure it at 20mm is if you ignore the half of the casing that has the AA batteries in it!