MYOG “Bottom” quilt that zips into mummy bag

I am not sure I follow exactly what you have in mind.

During winter, the two Marmots go together. Basically, you would like to make a replacement for the Sawtooth so that the replacement and the Helium are used together when it is warmer. ?

Sounds like a good idea to me. The 5oz quilt should be pretty easy to make if you have some sewing experience, room to work (table), and if you don't have much sewing experience they are still pretty easy.

There are some posts on here that say 5oz insulation is good for 20* F. I think these are mostly older posts. I haven't really found that to be the case. If the Helium will stay mostly on top and the 5oz is to give a little more width and tucking under, it sounds like a good weight.

If you leave plain nylon underneath, you would have the equivalent to a top bag. That is also a good idea. I figure you have a couple sleeping mats together and the nylon panel would lie on top of that. How wide to make the panel and etc. depends on the mats you use and who knows what else. How much moving around, equal sharing of sleeping space, stuff you have to decide.

I would think about making the strip more narrow at the foot. Chances are, the whole construction will taper at the foot and the panel should as well, I think. If you don't like the panel, you could go back and add insulation to it.

Oh, there are many places to buy zipper pulls and bulk zipper tape. If you have to go hunting for a zipper to match your Marmot, Rain Shed does a good job.

Mmm, if you use 2-way pulls, you could leave the combo zipped at the top to stay on and open at the foot.

My first post was kinda convoluted, but you got the idea exactly. I'm looking to make something that would replace the Sawtooth during warmer weather, and be a lot lighter than the Sawtooth, which weighs in at 55 or 56oz.

It could definitely be tapered down by the feet. We usually sleep with both of our feet inside the one footbox of the Helium. I like the idea of making an appropriately sized nylon bottom, and then adding any insulation that might be needed. Thanks.