Well, wish i had the skills to make a quilt with these combined within the same layer, but i'm not sure that is the case when it comes to down. In any case, i already have a few down quilts (well technically one is my wife's that i bought for her, but what's her is mine especially backpacking gear….)
So, i thought about a quick and easy way to combine these, and have a flexible, adaptable system. My main quilt is only a 40 degree one. What i plan to do, is to sew some fabric on one side, leaving it open at the wide end with the fabric a bit longer than the quilt and sew a few strips of velco on the top of the quilt and on the flap end of the fabric.
Cut some 2.5 oz Apex to size, put in the "pocket", flip the flap over, velcro and voila, secure the Apex material. Just added about 15 degrees and a little security should i experience some down collapse. I can take out or add in the Apex according to whim or need. I can also use that pocket when not using the Apex for when i want to cut down convective heat loss.
Since i will be ordering more Apex from thru hiker, figured i probably should just go with the lightest fabric they have there, which if i remember correctly is the M50 material.




