Not sure, but, if it helps, nearly any 10 speed rear wheel can be easily made 11 speed compatible via the solutions and mods here:

https://www.darkspeedworks.com/blog-11speed.htm

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I could do it by running an 11s cassette as a 10s, I suppose. I never need an 11t anyway.

But machining anything down isn’t an option on my old zipp 303 tubs, unless I get them rebuilt with new hubs (which I’m thinking about doing anyway).

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If look at that link, you will discover that you don't need to machine or touch your old zipp hubs, and actually that is not a good way to go (modifying 10 speed freehubs this way can cause the freehub lip to fail).

However, any machining that is typically done is done instead on the 11 speed cassette and that mod does not harm the cassette in any way (no loss in strength, durability, or function). Actually, it makes them fit flawlessly on 10 speed hubs, even very old zipp ones.

And there is a guy in Canada who does a great job on this mod and the prices are pretty reasonable too (see that link for more info).

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You could use one of the methods above to use an 11s cassette, remove one of the cogs, and set the Limit screws on your 11s derailleur to prevent over shifting. This is by far the cheapest approach.

You could also take a lathe to the largest sprocket on an 11s cassette to remove ~1.85mm that allows for clearance for all 11 cogs. Explained by Shane Miller here. My father has a friend with a lathe and it worked quite well for me. You can also buy cassettes with this process completed here from a guy in Quebec.

If you want to mess around with your di2, you could switch to 10s 6770 front and rear derailleurs. You've got to switch both as they are not cross compatible with 11s derailleurs (the shifters, however, are).

Don't mess around with the thickness of the free hub bodies on your Zipps. They're thin and are likely to crack.

I don’t have a pic here, but on the particular zipp hubs I have there is less than 1mm room between the lip on the cassette body and the hub shell. If nearly 2 mm is machined away from the cassette, then it would interfere with the hub shell.

I could use the 11-32 Ultegra cassette, I suppose, but I don’t really need to run a 32 cassette and would prefer tighter spacing.

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The Ultegra CS-R8000 cassette 11-32 requires an 11 speed freehub body.

The CS-HG800 cassette 11-34 will work with a 10 speed freehub body.

Right, thanks for the clarification!

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If you don't want to machine cassettes or freehubs, your best option is to do the remove a cog option as indicated in Dark Speed's blog. This has worked perfectly for 11 speed in my case (although I have machined hubs too with no problems). Another option is to go di2 with a 10 speed (someone correct me if I am wrong) di2 rear derailleur but good luck finding those new ( a few used options on ebay). I was in the same boat with 10 speed on my TT bike but finding 10 speed Ultegra or Dura Ace derailleus was becoming harder and harder.