Step 1

Use side cutters to make the tab less tall

Use small pieces of  plastic card leftovers from other projects
Glue these pieces on to the coins/mdf bases to make a suitably wide slot for the tab

 Glue the
pieces off centre to get a more balanced look when the
figure is mounted on it's new base
 

Glue the figure into place

 The figure won't stand any higher than a
figure with an integral base glued on a coin

Now base like you would any
non-slotta figure