Which downtube friction shifters work with 2021 Campagnolo 11-speed cassettes, and how well?

Friction shifters are infinitely adjustable - by definition they don't have stops or detents for each gear position. If they did, they'd be called "indexed"

So there are only three attributes your friction shifters need to run effectively

1. Total range - the lever must be able to pull enough cable to actuate the mechanism across the whole cassette.
2. Accuracy of your fingers/ears - The rider has to be able to place the chain in-gear without rub, while riding. On a 5 speed block there's a lot more space between cogs, and the clatter is obvious. For a 12 speed block, the cogs are closer together so the sweet-spot is smaller. Can you find it by feel while riding?
3. Resistance - the friction lever needs to be able to hold its position and not drift, which would give you a spontaneous up-shift.

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An addendum - the frame/bars needs some way to hold your friction shifters. Historically that was a set of brazed-on mounts, but a modern frame won't have them so you'll need to use a band-on set of mounts that can fit around your down tube.

Stem shifters are another option, and bar-end shifters are also available in indexed or friction mode.