Putting a pillow over the air intake of a diesel to stop it is very risky. The suction is imense. It could suck the stuffing out of the pillow into the engine requiring at least a teardown to clean it up. Many diesel have no air filter and anything put in the intake could be ingested.

A far safer method would be to shut off the fuel. The worst downside of that would be having to bleed the fuel system.

Even if there is no manual stop you should be able to reach the solenoid linkage and pull it. WATCH OUT FOR ANY NEARBY BELTS.