I hesitated to reply since I'm not an expert, but I do know that when we put in our vermiculite/cement sides and bottom, the angle on the walls and hopper sides made it easier to to lay down the mixture. The mixture has to be just the right consistency, and it has to go down pretty quickly or else it won't adhere to the dirt base or it will just slide down.

The straighter the walls, the more gravity will pull down the vermiculite/cement mixture, the harder for it to stay where the installer puts it.

We have a 10.5' deep end, and the angles are fine. Jo