I wanted to share how I make my dirt Bases. It is easy and cheap to do. There are many ways to do this, this is mine. Here is a picture of the finished base so you don't have to scroll to see what we are making.
First select a piece of plywood large enough for your base. I got this piece from a new house construction site, they throw tons of smaller pieces away on each job.
Straighten the wires on your form as much as possible. Slip a piece of paper over the wires and mark where they come through the paper with a marker.
Transfer the marks to the plywood where you want to attach the form and drill holes large enough to accommodate the wires.
Place the form on the plywood to insure the holes are in the correct spot.
Mark the shape of the base you want and cut it out with a Jig saw.
Route the edges of both sides with a round over bit to remove the sharp edges.
Place the base on a sheet of plastic to catch the mess. I use dry cleaning bags cut in half to make 2 sheets, but a trash bag will work fine. Without this plastic, the base will stick to your bench.
Pour about 8 oz (1cup) of 2 part foam over the top of the plywood. This will vary with the size of the base.
As it rises, place another plastic sheet over the top of the foam. Gently spread the foam around to cover the edges. Pat it down to control the height, you can trim the edges later. Allow it to set up.
It is hard to tell what is going on in this picture, but I am removing the top plastic sheet. Also, separate the base from the bottom sheet but leave it on the plastic.
Trim the edges to the shape you desire. Scuff up the top a little.
Now paint it with a dark brown latex to help hide any thin spots on your finished base. Allow the base to completely dry.
Once dry, generously coat the entire base with Elmer's glue or Modge Podge.
Next we add the dirt. I used plain old ditch dirt, any dirt will work. Dry it completely and sift it. Completely cover the base with the dirt. It is better to use to much than not enough. Pat the dirt down to insure good adhesion.
Lift the edges of bottom plastic to get good coverage on the edges.
Carefully flip the base over to remove excess dirt. Spray the entire base with a 50/50 mixture of Elmer's glue and water. This is the seal coat so complete coverage is important.
Remove the base from the bottom plastic and allow it to completely dry. I like to place them in the sun to dry them quickly. You can mess it up very easily while still wet.
Again, here is the finished base. You can add plants or what ever you like. I hope this helps.