I know this is after the fact but if you haven't done all of your "plaster cloth" then the best way to use the stuff on the foam risers is to lay it on top of the riser and drape it down either side - no overlaps on top of the riser that way
A possible solution would be to sand the overlap flat then place another piece of plaster cloth over the entire riser. That should give you a flat surface to work with.
With all of that being said, I think you are pretty much worrying about nothing though. Unless the plaster cloth has wrinkled badly, the very small little ridge an overlapping of the cloth would cause would not effect the roadbed, especially if using cork roadbed. Frankly, I wouldn't be concerned but would lay a slightly thicker spread of adhesive for the cork roadbed.