I bought a wooden rocker plate with fore-aft motion from ‘TrainerTweaks’ on Etsy. It works with my Kickr Core and several other models.
What’s cool is that it uses a design that doesn’t need any inflation (though you could add it if you wanted) and is very easy to assemble. He gives you everything you need plus video instructions on YouTube.
Having ridden it for a while now, I can say it is great for reducing saddle discomfort and feeling more engaged. But it doesn’t require constant attention like rollers do. Just be sure to balance your setup at first then the on-center stability is really nice.
The other nice thing is that it’s very compact. It is only a bit larger than the Kickr Core. The front wheel just sits on a little block. So I can easily put this away in a closet the few times a year when I need to put the trainer setup away.