I have a trip coming up this weekend(read that as 'no time to order anything')... And I have a 40* shamu that I have taken down to 45 with comfort, my topquilt is a down sleeping bag that is of unknown temperature rating but it should suffice...

How would you go about stretching the shamu down that far... I have a mylar blanket that I know I can add, and the hot water bottle trick, and I always have some hand warmers... Should this be enough or is there another option that I should carry, weight is not that much of an issue to me... I carry 35 lb pack regularly to work as my workout and excercise and my pack weight right now is 30ish including water food and fuel... So a few pounds of security is ok with me..