There are a few 2-door pet carriers that say they can handle 20 lbs. If you leave the carrier out (without the front door on) and feed/treat the cats in it, they may get used to it a little bit more. I'm guessing it's not possible to get your cat into something like a pillowcase where you could tie it shut and that could be put into the sedation box at the vet's. Are they able to "squish" him down in his carrier and give him an injection of sedative into his muscle? That might be less stressful than trying to gas him down.

https://www.chewy.com/petmate-two-do...w&gclsrc=aw.ds

https://www.cat-in-the-bag.com/

http://www.thepurrcompany.com/shoppi...php?prodnum=24