The first casket was fiberglass from the prop room that they cut it in half and lengthend it from what I understand. found pictures but cat find the article. The next 3 were coffins that were purchased in Mexico. Heres the article with one of the builders.
http://www.munsterkoach.com/dean1.htm
http://www.munsterkoach.com/production.htm
theres nice old pictures of how they did it back in the day.