First of all, do not kennel cats under pretty much any circumstance. They're not dogs and it will only succeed in making anxiety levels get worse.
Yeah see that was my first thought too (to be fair I thought that about crate training dogs too when I first heard of that) but strangely I've read a lot about it being used for anxious cats with great success, and it got me curious if anyone here knew anything about it.
We've tried pet remedy (valerian) and feliway but they don't seem to have much effect on her, so I've just bought some canicomplete/rescue rememdy to try next as it seems to have good reviews. Also got her booked in to the vets this weekend to check the areas she's overgroomed and see if there's anything medical going on (eg allergies).
Part of me is hoping the younger one is just young and playful and will grow out of it soon (or start to read the signals at least) but I do worry that's she's starting to do it as some sort of challenge.