how do i make an entity to spawn

Structure spawning mechanics are pretty hard coded, as far as I can tell. There's a couple possible solutions:

• Have the entity check for blocks of the structure nearby, using the 'check for block in 6*6 box' template procedure. (Have it check for nether bricks, for example.) This would effectively solve the problem, but would raise the issue of farming mobs by this method.
• ...Or my preferred method, make a special block/entity, (it can be a collectable part of the structure, or something invisible that can't be altered), that generates in your structure, and spawns enemies on available spots nearby until it's deactivated.