I use a Montane Photon teeny packable jacket, i bought it last year from CRC cheap(ish) and within a week i had bought another one as it is that good, I'm between a 38"/40" and a medium is a perfect fit, here's the montane bumpfh from their website.
Design a waterproof bike shell with exceptional breathability and light enough to pack into a jersey pocket was the brief. The Photon exceeds this with a breathability rating of a startling 25,000 MVTR and a phenomenal weight of 190g. The Photon revolutionises weather protective bike clothing.
Exceptionally high fabric breathability at 25,000 MVTR and a 20,000mm hydrostatic head
Bike specific tailoring to reduce volume at the front of the body and a long drop tail
Upside down chest pocket zipper so that gravity aids closure whilst riding
Rear shockcord collar adjustment to prevent rain entry and core body heat loss
2.7 metres of 360° Dusk till Dawn reflective body contoured piping
Internal MP3 accessible headphone pipe
Water-resistant zips with semi automatic puller
Moulded cuff grab tabs for ease of use whilst riding
Includes stow away stuff sac
As with all MONTANE® clothing PERTEX® Fabric contains a Durable Water Repellency (DWR) as the first barrier to keeping you dry. Clean your garment in order to maintain this.
Machine wash in warm water at 40C/104F with a mild soap. MONTANE® recommends that you use Nikwax Tech Wash.
Montane Photon here