sfall XYZIN model.pdb HKLOUT model.mtz << END-sfall
MODE SFCALC XYZIN
CELL 15 20 25 90 90 90
SYMM P212121
RESO 30 2.1
LABO FC=FC PHIC=PHIC
END-sfall

Here is another one:

iotbx.pdb.as_xray_structure model.pdb --fake=2.5

Faster than phenix.fmodel, but no bulk-solvent correction.

Ralf