Here the base has been filled and sanded. I use apoxy sculpt putty for my main filler (that's the blue stuff) which has a texture somewhat like green stuff and roughly the same hardness as plastic-card so they sand similarly. I then use squadron green filler for a much smoother finish and to fill smaller gaps/holes. I also find that the difference in colour is very helpful when sanding as you can see where dips and hollows are.
In the above picture you can also see the holes in the base where the legs of the copter undercarriage go though the skids and into the base. This was the best way I could think of for making the copter secure on the base but it is fiddly to construct.
I need to turn the copter a little to its left. The skids need to extend over the edge of the pavement a little more for composition reasons, and I'm also not happy that the skids follow the lines of the pavement - it looks too contrived - so I'll have to drill another pair of leg holes and fill the existing ones.