My vet cut a collar out of clear plastic. You cut a washer bigger than you need with a large center hole. You then cut a pie shaped slit to the center and be careful to round the sharp edges. Then when you pull the edges together it will form a cone.
The most important thing is wrap all cut edges with 1/2 inch wide medical (Johnson & Johnson) white cloth tape (to protect the neck hole), and to close the collar and hold it together. leave it lose around the neck, only tight enough to keep it from slipping over the head. My little sick boy had it on for 25 days.
In 2011 Moonbeam had a staph infection for 11 months and would pick under his wing till he almost bleed to death.
Today he is alive and well, he can't fly but is still happy and healthy.
He had (Staphylococcus intermedius) or commonly called (MRSA Methicillin-resistant Staphylococcus aureus) the skin eating infection that changed to the avian strain Staphylococcus p.
He picked it up from his previous owner, whom picked it up in a hospital.
I gave him intramuscular injections of Amikacin (twice a day for 10 days) in his breast!
He took it better than I did.