If you have the Series VI hood, you have a Series VI adapter and can now use Series VI filters, which are somewhat more readily available than 40mm-fine filters.
Canon made both round and vented-rectangular hood styles in this size. The rectangular model is from the Serenar era, labeled "1:1.9 50mm, 1:3.5 35mm" and has a 42mm slipon mount .. so would also fit any of 50/1.8, 35/2.8, 35/3.5 and possibly also the 35/1.8.
For current examples on ebay, search for
"Canon lens hood/shade for 50/1.9 and 35/3.5 Serenar" (rectangular, slipon)
"Canon RF Chrome Hood for 50mm f1.8,35mm f2.8,35mm f3.2" (round, threaded)
... etc. Rectangular models seem to go for much less ($35 vs $100?) than round ones.
As noted earlier, the 35/1.8 has a deeply recessed front element and is already well protected from flare, and its possible that adding one of these hoods may vignette the lens (this might explain why Canon didn't offer a matching hood). I have no idea. Try it and let us know 🙂