So the mechanics here are fairly simple: it's a weaker version of an immovable rod that has a fudgeable time limit. What do I mean by fudgeable? The time limit is tied to it overheating, not it running out of fuel, which means you can let your players gamble, by letting it run hot in a pinch. They might burn themselves, they might pick up a new scar or two. Or they might turn the shield into an IED. Who knows?