Miriam,
If you want to make your own, I would suggest starting with a quilt with a boxed foot and build it out of Climasheild.
Its easy to work with and you only have to sew it on the edges. For fabric hit the local Walmart and you might luck out and find some 1.1 oz DWR ripstop. I found some a while back for $1.50 per yard.
If you cant find that go to Thruhiker and get some .9 oz nylon there. The taffetta is supposed to be really comfy.
http://thru-hiker.com/materials/breathable.php
Use 2.5 oz Climasheild for summer and about 50dF
3.7 oz for 40dF
5 oz for 30dF
Etc
They are out of 2.5 oz right now.
http://thru-hiker.com/materials/insulation.php
If you do a drawstring footbox its pretty easy.
A real footbox is a little more difficult but not too bad
Basically just sew a shell inside out, edge stich on the climasheild,
turn it right side out and close it up.
If you want to build a down sleeping bag or quilt its a lot more work
with boxed channels etc.
I have worked with down before and its messy.