Whatever he/she does......don't give benadryl or any other sleep aid to the cat to sedate him. Dangerous!!
I have found that top loading carriers work best. Probably more expensive than you're looking for. My cat knows that carrier = vet. After they learn that, it's almost hopeless. Try putting a shirt or something that has the owners scent on it in the carrier first along with any toys or things the cat might like. This might calm him down. Right now the poor cat doesn't know anything except his owner left him with somebody who is trying to stuff him into a box to go to the vet! I would be mad too. Calm the cat as much as you can first. Good luck!
I find that if you tilt the carrier almost so it is sitting straight up, and ease the cat in bottom first, that works best. Make sure that you have a firm grip on both front legs and back legs to avoid getting clawed. I have a cat that HATES the carrier, but hates the car worse. This works best for me, let me know if this helps :)
I dont know about what you can get over the counter as I dont know what area you are from,but i would say you should call the vet and get advice.
Yikes! I agree with Tigereye. I don't believe you can get sedatives like that to knock the cat out and I don't believe that's the right thing to do (personal opinion). Try the towel/blanket thing!
Sorry, but how can a cat claw through a towel? Maybe you need a thick blanket and a different approach. Get a thicker blanket or towel, throw it over the whole cat, head included, bundle him up so he can't stick any paws or his head out, and stick him in the crate. This does work if you do it right. Even if there was something over the counter, I wouldn't think you'd even be able to give it to him if he's as bad as you say. Try the towel/blanket again and maybe a bigger crate