Before I got a fine enough plastic strainer (it should be small enough to hold flax seeds, basically), I used a plastic fork to hold back the grains and pour off the kefir liquid into another container. Just keep an eye out for any escapees. You might be able to use wood, but I think wood is more likely to hold onto mold than plastic.
As for using a plastic jar, I personally would not. I’m sure someone you know will use a glass jar of marinara sauce soon, and you can just wash it out and use it.