Kelischek Workshop will put plastic keys on a Yamaha or Aulos tenor barrel for $35/key: link:
https://www.susato.com/collections/comfort-key-retrofits
I had it done and they are quite good. If your tenor recorder is not either of those brands, you could probably call them and ask them about your brand. You have to send them the barrel anyway (even with Aulos and Yamaha), so maybe they can do it for you with a different brand (perhaps somewhat more money?)
