Checking e way guys do those engine manifold can heaters, I don't think they pop the lids. If the can bulges then it bulges, but nobody wants engine compartment fumes in their food. You wear gloves anyway when handling the can. I have looked into something complex that works as a heat exchanger off the headers but the people at the parts store say that anything on the headers that interferes with heat dissipation can make the mild steel pipe go brittle and bare headers last longer. I am looking at copper tube wrapping that passes to a fluid and heat exchanger reservoir.

In the past, when the heater was stuck in the on position anyway, I just stuck MRE packets up on the dashboard and after an hour or so they are just right, but it's not enough heat to warm up canned food. It is enough to thaw stuff out and make the microwave more efficient.