Coleman - Kompact™ 40°F Rectangle Sleeping Bag

Honestly, the Galactic is kinda stretching it as a 30 degree bag. Another would be just about perfect at 40-60 degrees. And being down, it will have a larger comfort window than synthetic options. I've used a 30 degree down bag in temperatures from mid 30s to 60s without feeling too hot, and I sleep fairly warm. I don't think you will find any down options that will zip to a Galactic, and if you found a compatible synthetic 40-50 degree bag, it would probably weigh 50% more, and maybe not be any more comfortable in the 60-70 degree range. But to actually answer your question, you'll need to find a bag with similar dimensions (Doesn't have to be exact, but a mismatch will mean the corners won't line up, and you'll have one edge with extra bag hanging off), and it will need a matching zipper (Usually YKK, probably a #5 or #8 on the galactic.). I can check the zipper size on one of mine tonight. Edit: Found this one that looks like it matches dimensions, and probably has a #5 zipper. It's cheap, and pretty lightweight. I think it matches the current Galactic at 33"x75", but they had 4-5 different color / size / down fill options over the years, so check yours if it's not the current version. [https://www.sierra.com/coleman-40-f-kompact-sleeping-bag-rectangular\~p\~6kfkn/](https://www.sierra.com/coleman-40-f-kompact-sleeping-bag-rectangular~p~6kfkn/) I couldn't find a weight anywhere, but probably still heavier than a Galactic.

I agree this is the way. Two bags. Originally bought a bag rated for -10c but woke up shivering in the middle of the night of -6c. Now I use a Coleman rectangular bag over my -6c down bag. Makes a world of difference. Plus that with the double bag system you can use just the one in more warmer temps.

Most rectangular Coleman sleeping bags can be zipped together and won't break the bank.