You can get base toppers which go in the inside but then you need snip off the slotta tab drill and use pin to connect the figure to the topper.
With mine
1, Glue the figures slotta so it sticks out the slot about 1mm so its more or less level with rim of the base. Let it Dry.
2, I then push Milliput from underneath and then scrape its so its level. (Doesn't matter if it bulges slightly as you can sand it when dry)
3, I then fill in the base from the top with milliput. Dip a knife in water so its wet then scrape it across the top to get a smooth flat finish.
4, Wet the knife and then gently score it in slab/cobble pattern.
If the figure has an integral bases you can still do the same in the base is a bit thick just file it down so its level.