The Invisible flag is from old times, it used to be what you want to have.
Sadly, when (the nevertheless) good c_move replaced ent_move, invisible blocks where not compatible anymore.
Is there a solution in WED?
You have to create your boundary, extract it from the map - f.i. group the blocks, copy them, new level, paste, save, build - and import the blocks as a map entity and set this one invisible.
