I almost went with a solar reflective umbrella at 1st. because I like everything I carry to have dual or multiple purposes, rain/sun. Finally, I decided on the Senz XXL umbrella for its size, durability and that the design allows me to use it while moving on the bike. It's unique and very well made.

https://www.senz.com/en/stick-umbrellas/senz-xxl/

I have used it to collect water and to support a Cuben Fiber/Dyneema Trail Star tarp/tent too. So Multi-purpose! Did you know rain has Vitamin B-12 in it? That's why milk has B-12.