For commuting, you need an extra-long seatpost. Most normal seatposts max out at 400mm in length. To commute on a BMX, we need to go longer.
For comparison, here are some folding-bike seatposts:
| model | diameter | length |
|---|---|---|
| Brompton | 31.8mm | 530mm |
| Dahon | 39.3mm | 650mm |
| Tern | 33.9mm | 585mm |
Brands known for making longer-than-average seatposts: