ItsThatGuy,
Did I read previously that you attached the front and back already, and are looking to set the dome now? I went back and scanned the previous posts, but couldn't find the comment so maybe I'm making it up. However, if I'm not, I'm wondering if you couldn't use a version of what you described above. What kind of opening are we talking about, between the dome and the rim? I had a bit to fill, but I'd be surprised if it were more than a 16th or an 8th (and an 8th seems like a whole lot to me), but I could be wrong. I was thinking you might use that foam as a temporary ledge to set the dome on, provided the front and back are together already. Instead of affixing the foam to the dome, make a ledge of the stuff on the inside of the helmet and set the dome on top of that. You could then fill the space with bondo (basically what I did), but not if we're talking a great distance. If we are talking a big difference, then I would consider separating the halves, taking off some of the under-the-ear sections, and trying again for a snugger fit on the dome. If the front and back aren't already joined, then I'd put the front on the dome (cuz it's easier to center), and then the back on.