The answer to both, in my case: F-clamps. I love those things, especially those single-hand F-clamps, and I've used two tiny ones for years now. I should probably say "abused", though, because, well, they are not intended to be clamped down onto something upside down and then get stuff attached to the stem sticking upwards. There's high-quality ones that will withstand this kind of abuse, though, and cheap ones that will eventually give up and come loose with rather little tension on their stem. Which means the sensible thing is to go for the higher quality tool... with the higher price.
Unfortunately, regarding my two questions, "F-clamps" is not the full answer. The full answer is "a shitload of F-clamps", and it has taken me a while to get to terms with that, bite the bullet, and invest. But I have found a good clamp, they have arrived, and I'm now working on finalising my teaching script, getting the handout (which will be a how-to-make-your-pattern cheat sheet) all done and lined up, and then there will be three nice guinea pig ladies coming over next week to try their hand and my teaching ideas.
It's all quite, quite exciting - and if it all works as planned, the course will cover warping in the "endless" method, the basic system and mechanics underlying all tablet weaving, how to read and write patterns, and how to do 3/1 twill on tablets, including how to work (and design) patterns in this method.
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The course will then have its first proper appearance at the Nähtreff in October - which right now seems rather far in the future, but with the way time is flying, it will probably be happening before I know it!