Since these models, and most other Tyranids... fall over when looked at funny, I've decided to glue 1" metal washers to the bottoms of all the slottas for the stealers. It makes them far more stable. I considers slicing pennies but there isn't enough heft compared to the washers.
I tried the pennies with some Hormagaunts, and found them wanting. Instead of using washers, I've been filling the slottas with Milliput -- seems to work okay.