It's an easy assumption that any vessel subject to Zero-G and/or changing positive-G will have tether and load-handing tie-downs. It's also an easy assume that Vacc-suits and Battle Dress will have semi-standard harnesses with retractable tethers and some form of connector. Each tether will secure the character in a single dimension. Two tethers and a hard surface will pretty well "glue" a shooter to that surface like a rock-climber bracing against a cliff face.
Assume one combat round to fasten each tether to a tie-down within arm's reach, with the first tether providing +1, the second tether providing +3, and the third tether providing +5 to the "save" target number. The same modifier would also work for attacking someone who is tethered; making tethering without other cover a risky move.
Naturally, characters using more than one tether will effectively be anchored into place and will not be assaulting. However, defenders, snipers and support weapon operators would benefit from being able to fire without having to deal with spinning out of control.
For those who are interested: Zero-G shooters, even if well anchored, will experience enhanced recoil from their weapons. Calculations of recoil energy, factor in the gravitational constant on the weapons mass as a suppressing factor on recoil. This, of course, is not available in free-fall although the weapon's static mass will still retard some of the recoil energy.