Question on Layers

I have a board for a game I’m building and when I place terrain around the edges of the board sometime the terrain sticks over the edge. I have tried to build a layer around the edge of the map that would “hide” the stuff hanging over but when I do that I can’t go back and select the terrain because the “Terrain Mask” layer is above the Terrain Layers. I know I can fiddle with the Layer stack but then the “Mask” wont hide the terrain. I know this can be done because I have seen it in other modules. I have tried to copy the effect but have had no success. Pleas help.