You are correct that bees do much better in a tree than conventional bee hives, but not because of the shape, but the insulation value. You have made your cavity much too big for the bees to get full benefit. The tree walls should be 4-6" think. Check out Derel Mitchell's research on the subject which also discusses Tom Seeley's findings on natural tree cavity size.

https://docs.google.com/viewer?a=v&pid=forums&srcid=MTQ1OTYwMzIwNzQ2NzIzNDE4MDcBMDc3NDA4MjkyOTA0MTMzNTYzMDUBUjE0WWxzVU9BZ0FKATAuMS4xAQF2Mg&authuser=0