The best way is to fix problem, not use some enforcers and hacks.
First of all, check wheather your map is inside map grid. To show map grid go to Optimization menu and Show Objects Grid. If your map is not in the grid - resize grid in level attributes.
Open Objects window, go to very top and select level. Next in Attributes window go to Terrain tab and in objects grid sizes section change values for X, Y, Z fields.
If this will not help, we will try to find other solutions to this problem. You should use NavMesh enforcer only if it is necessary.
Also attach some screenshot of area without navmesh.