If it is just one shotgun that you need to secure, maybe a safe isn't the best approach ... you might consider a locking wall mount like this:
http://www.mossberg.com/content.asp?id=519
They are under $50 and if you put it on a sturdy wall (again the Mobile home might be an issue), they are pretty solid and you can hang a jacket on them and they just look like coat rack.
That would free you up to look at more cube shaped boxes for your handguns. On that note, I'd seriously consider leaving half of the pistols with your father ... worst case, you end up with an excuse to go see the folks.
Saands