Why not simply make it a large stone wall with a plaster type finish, this way you are not restricted to scale sized bricks and you can use cork from wine or champagne bottles broken up and filed to accomodate a decent finish.

The picture is to give you an idea for composition, the finish is down to your own artistic creation, you could do half a wall without the need for the complexities of the hole. It would still look very good.

Alternatively, pick rough scale stones from your garden, let them go bone dry, glue together and add your plaster.

Have a bit of experimenting first but as already stated there are so many ways to skin a cat!!