In mission six: the unknown lifeforms from the Deathwing expansion the marines have access to powerfield generators. A powerfield generator is a small device that generates an invisible force shield. It is used for sealing hull breaches in emergency situations, or can be used to delay enemies by blocking off corridors. It takes genestealers 18 action-points to destroy the generator.
I've made 20 of them, 5 for each marine chapter that I'm going to paint (of which Space Wolves and Imperial Fists are allready done).
I placed transparant plastic that I cut to the size of a door on small bases. This didn't give the look I wanted, so I tried to paint some reflection on the supposed force field.