In a top-bar hive, what angle do the side walls need to be at so the bees do not attach them to the bars?

What it says on the tin, really. Yesterday I made a post about a oil drum hive, and some of you raised the point that the side walls of the top-bar hive need to be sufficiently angled so that the bees treat it as the floor rather than the walls and do not attach comb to it. To be honest, I never actually thought about why the sides of the top-bar beehive are angled.

Onto the question. What angle do the sides of the top-bar beehive need to be? is there anything that can be done to make the bees less likely to attack comb to the sides?