I bought a Marmot Nanowave sleeping bag for bike touring because it is inexpensive, lightweight, packs down very small, and is made with synthetic fibers that will stay warm when wet. It retails for $80-90 at REI but you can sometimes buy it on sale for less. It comes with a compression bag and packs down very small, which is important for fitting in panniers. It is rated to 45 F, so I also bought a liner than it is supposed to extend the comfort range by about 10 degrees.

I've only used the bag a few times but it has worked well so far.

https://www.rei.com/product/846001/ma...e-sleeping-bag

polar fleece blankets would work . just not so compressible ..