One PBS member uses soda bottles as space holders, allowing soil to filter between them for additional root run and hopefully avoiding a perched water table. My concern is that as soil filters into the cracks and crevices there is surface subsidence. If the only intention is to A) reduce volume of expensive potting mix and B) reduce weight while still maintaining root run, why not just shove a smaller pot upside-down into the larger container. That provides a central "core" of air while allowing a "jacket" of soil between the larger container and the inverted smaller one. The soil jacket is easy to fill when repotting, and it will be easy to remove the bulbs for repotting, propagation, whatever. Relative size of the two containers adjustable, so options are available. Judy in New Jersey where the wind is shaking tree tops like a rehearsal for the lion-like start of March