Parents with a Tripp trapp chair: any hacks to keep the baby from climbing out?

We have a Tripp trapp chair and I purchased the baby set from REBEL. When it arrived, it was missing the harness. They were out of stock of the item so they offered me a full store credit refund and just allowed me to keep the other parts. I figured it wasn’t a big deal and found a harness on fb marketplace.

We have been using it for a few months and it’s worked great, but now my 9 month old pulls his feet until the feet holes and stands up on the seat and climbs out. I tried the harness that we bought on marketplace but it’s not compatible with the new baby set. I have searched everywhere and cannot find a single harness for sale that is compatible with the baby2 set. (In the US).

We have had success with stuffing a towel behind the baby so he can’t pull his feet in but I would reality prefer to have the actual harness. Stokke website doesn’t have them. Secondhand sights only have the old one. I haven’t found any off brand compatible ones except for a sketchy $6.00 Ali express one that keeps coming up in my search results.

If you know where to buy this harness or any other solution that works, please let me know. He’s wearing me out!