You've made the classic mistake of a first time forest--You tried to make a "forest" not a composition
. A forest, as most people are used to see it, is a space where trees are spread pretty evenly and regularly.
A bonsai forest is the opposite. It uses space differently. Trees must be tightly and asymetrically grouped, using open space to accent them. That means tight grouping of trees in one area, leaving a balancing area of open space in another, or a series of "tree spaces" punctuated by "open spaces."
Simply planting these trees to one side and as close together as you can get them, using trunk thicknesses as a guide to which to feature in the front, will make it a great one.
The slab you've chosen is an excellent one for a forest. Slabs are the ultimate pot for forest plantings, as they emphasize space and openess, yet provide a foundation for the trees to rest on.