I ended up buying a bosu ball a couple years ago, after seeing a PT for some back and hip flexor pain/instability issues, and I knew I had some muscle imbalance issues (due to the great majority of mu exercise being linear on the bike). Turned out I was right, the imbalance issue was bigger than I realized.
My Bosu ball was great, but i ended up with a leak. Anyway, I bought a much cheaper round plywood wobble board with plastic center, that functions somewhat similarly (though to a much lesser degree) than the bosu). It does a pretty good job for beginner level balance work, and for about $25. AND, it would be a lot harder to break your neck, due to it’s more limited range of motion, for those worried about that.
like this: https://www.amazon.com/AmazonBasics-Wobble-Balance-Board-Black/dp/B07DWKNXNY/ref=sr_1_1_ffob_sspa?crid=2B8AQVZ47Y9CO&keywords=balance+board&qid=1688560945&sprefix=balance+board%2Caps%2C647&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1