Welcome to the forum!

I haven't transported a snake anywhere except home from the petstore, so far...and they gave me a paper bag to do it in. ....But from what I hear, a cloth sack (such as a pillow-case) that is very securely tied at the top is one of the least stressful ways to transport a snake. If I were doing it, I would put the sack into a secured cage as well, like the critter-keeper Marla mentioned. Your primary goal should be escape-proof! No matter what you do, the trip will be stressful for him...but sometimes we have to find the lesser of the evils. After the trip to the vet, put him back into his home and just let him chill for a couple weeks without any handling other than absolutely necessary.