I built mine out of parts I had on hand. It's rear wheel only, but I may extend it to support the front wheel too. It has fore-aft motion on round linear bearings. The linear bearings I had were bigger than the typical 20mm, so there was a bit more space between the plates than most people have. I wasn't happy how floppy it was with the playground balls I was using. I saw someone used a couple of pilates balls that are a bigger diameter, and so I tried those and it works much better. I probably could have added some spacers to change how much room the balls had between the plates.
I have a kickr and I dropped the wheel size all the way to make it so I'm not going downhill all the time.
I wasn't happy with it last week and stopped the motion for a couple of rides. It feels really weird to have the motion stopped like that. I do think the thing I noticed most was the lack of side to side motion, but it seems like fore aft motion is important too. I used to have tailbone pain after about an hour before I put it on the rocker, and that has stopped almost entirely. I did have tailbone pain on one ride after I hadn't been riding for a while, but that hasn't recurred.