If all else fails, clothes basket plunked over the top of him ... would he willingly sit in a clothes basket if it had towels in it? And then be okay being carried in it? Maybe try that, and carrying him around the house for "fun" a few times so he gets used to it, then just figuring out a secure cover for it* - it's larger than a traditional carrier, so he need not feel as trapped ...

*cardboard and a couple bungie cords, it only has to survive one trip to and fro ...