That packing foam is a very useful thing to have lying about.

There are a couple of things that're also very useful along the same lines. People build foamies with insulating foam, the rigid pink or blue sheet foam. I bought a sheet a year or so ago and am still making things with it. Didn't buy it to make foamies, and haven't done one yet, but have almost used it up. Almost every ARF I've assembled since buying the foam has some of it in them, and I've assemble more than a dozen. And it's useful for other things, like airplane stands.

I also bought some furniture cushion foam awhile back. I got the densest I could find. It's still very lightweight for model use. I cut it to fit with some compression. It's in most of my ARF's that had free standing gas tank locations. They provided some sticks to glue around the tanks, and I tossed them in the "to be used later" bin. Cut the cushion foam for slightly oversize bulkheads and wedged 'em in. Got support that absorbs vibration.