Saran wrap. Wrap it around a couple times (depending on the effect you want add more or less) and tape it on. But make sure no other cows can get to it and start chewin' it. Or you can cover it with something, we have never covered it since we didn't have to deal with other cows tryin to chew it off.
Trash bags wrapped around the neck is what we use. Saran wrap works good. They can actually do better than the expensive ones. Just remember it will sweat hair out Alot faster than a real sweat. And it makes flakey skin on slick calves that looks HORRIBLe but you probley aren't down south and if your careful. Home made ones work perfectly fine
Eldon Miller had an article in the National Junior Livestock Exhibitor many years ago about using a section of truck tire inner tube. We cut out about a one foot section and slide it over their head. We used it on some old time simmi calves (lots of "leather" up front). The rubber keeps the hair and skin pretty damp so check them often so they don't get sores. When the calves got too big we switched to leather ones with a wool lining (you don't see these any more).
Depending on severity of sweeting needed....we've used icy hot, wrapped in seran wrap covered by the Sullivan's neck sweat. The icy hot doesn't cause much flakey skin.