I've no idea how deleDpro works but it may be simpler to leave your level as is and create the boundary in code using a simple cube like so ...
bound = createcube()
positionmesh bound 0,0,1
scalemesh bound, levelwidth, levelheight, leveldepth
flipmesh bound
entityalpha bound, 0
collisiontype bound, YourCollisionType
Stevie.
bound = createcube()
positionmesh bound 0,0,1
scalemesh bound, levelwidth, levelheight, leveldepth
flipmesh bound
entityalpha bound, 0
collisiontype bound, YourCollisionType
Stevie.