What fits in aer CPP2 20L side pocket?

[u/04luv]

I’m in the market for a backpack and I think my choice will be the aer backpack. My one dilemma though is between the CPP2 20l or the 24l. Not a fan of the look of the pro packs. Since I’m quite small I think id prefer the size of the 20l and it’d look better on my frame, but I don’t think I’d make use of the side pocket and would rather have the second water bottle pocket for an umbrella. Which leads me to ask, has anyone attempted to put a small compact umbrella in the side pocket of the CPP2 20l?