New frame has no seatpost – instead a thick lump of oddly shaped carbon rising 20cm from frame to a saddle (very stubby seatpost almost fully inserted). Any suggestions how I can fit a rear light? Perhaps you know of a very soft stretchy bracket of a light?

There are some that mount to the seat stay but only if that’s a conventional round shape.

You can get Cateye light brackets that clamp to the saddle rails.

On the back of your helmet

Depending on your position that’s a really shit place for it IMO.

Exposure Flare will mount on saddle rails.

As above some will mount on saddle rails.

IIRC the moon comet lights have a bracket to attach to the seat rails too.

Depending on the shape I’ve added some sugru to the tube clamp part of an elastic band mount to get them to fit awkward shape.

Something like this cervelo forum with picture

Mines on the seat stay as my saddle bag is in the way of the seat post

Look at the moon comet light; it comes with a seat rail mount

Something like this cervelo forum with picture

That would scare me, that band looks very taught, I had one break on a 27.2mm seatpost, and didn’t dare stretch it round the seat mast on my Top Fuel. I’d get the saddle rail clamp or something else personally.

Cateye RM-1 or Fizik saddle mounts are both excellent. Can’t fitanthin else to my aero seatpost