I am putting retaining wall blocks around my garden and would like to know what is the best base to put down underneath them and how deep should the base be? They are the 3!/2" high X 11 1/2" wide X 7" deep. I plan to go 2 high would you make the base 10" wide or 8" wide. I do not want them to sink into the ground. This is my first time doing something like this and want a very good job doing it. Any pointers would be appreciative.