I built one for a friend and he had me put an OS120FS in it. I installed the rudder and elevator servos behind the wing along with the battery. I mounted the receiver right at the wing trailing edge. I used the piece of wood that creates a hole in the bottom rear sheeting as a hatch and mounted the battery to it. I did not have to add any weight to it to get it balanced. It came out balanced at the front of the cg range.

I have attached a couple of pictures that show some of the mounting. The plane is covered with transparent green so you can see the servos, etc. through the covering. If you look carefully at the pictures, you will see that the servos are mounted at different heights for clearance purposes.

I thought it might help to see the entire right side of the plane so I added that pic too.

I hope this helps you.