If you've breached HFS and survived, congrats: Sladeland is yours for the taking. If you dump goblins into it, the wandering clowns will get 'em eventually. If you've made a breach but haven't called the clowns yet, I believe that tube is safe: clowns may wander in, but won't zerg you. Build a retracting bridge over the bottom of the tube (clever obsidian casting may be required to get there. Get goblins to it by any means necessary, then retract it to drop them in. You can do something similar with floor hatches, but if you want more than a 1x1 pit it will take more mechanisms. In either case, the crux of the matter is that building destroyers can't destroy buildings on a different z level.
Basically, the setup is to fill a room above the clown car with goblins, then make the floor go away, and finally quickly restore the floor. to be very safe, use an airlock system. Here's a cutaway
Fortress
| | < Tube. Perhaps put a mass-pitting room here?
|__| < retracting Bridge 1 (dump enemies here initially. Open only when 2 is closed)
|__|< retracting bridge 2 (open only when 1 is closed)
Clown Car
That's the basic idea. Remember, you're making horrific sacrifices to the denizens of the abyss, be stylish about it! Rose Gold constructions (I imagine it looking like blood-stained gold in this case, and the eerie light is purple), horrific engravings if you can manage, statues of dying goblins and other awfulness awesomeness.