There are a number of ways to extend the downward range of a bag, maybe even to reach it's advertised rating.... I just figure ratings all lie by 15 to 20 degrees on the wrong side and go from there.
Good warm coverings for your head and neck while you sleep will probably be the lightest and most compact addition you can do, such as a neck-gaiter and/or a neck-covering ski mask, balaclava or largish stocking cap that can be pulled well down.
Wearing additional clothes - such as thermal long johns, or even your (dry) hunting clothes or spare set to bed will help. I love putting on a thick pair of dry socks before crawling in - dunno if it helps in the heat department, but it surely feels decadent!
We've been out enough in marginal temp/bag conditions that now the Lab thinks he belongs in the bag with me... The Dachshund we used to have didn't ever know there was any other way.