1st off i know this pic is not the best so sorry. and pardon the mess in the room! but this is what i did.
This was my 1st attempt at making one. next one will be an under quilt thats more common, with the shock cord, and hopefully down.
ok here goes,
I took a 40* sleeping bag, unzipped it and folded it like it was closed up, sowed 4 pieces of webbing and done! easy and simple.
i knew it was going to get really cold so i took an old bed cover and simply "tacked" it in ( i actually used safety pins shhh dont tell anyone!)
It worked great at around 15* the other night, i stayed totally warm no chill at all! i was really surprised myself! i was contemplating going to the car if i got cold! but was warm as if i was at home! (not mentioning the top quilt and etc. but yes it worked great!
i saw the thread on here somewhere where i got the idea but couldn't find it again to send the link! so the second picture is of the idea i got!
i know it is heavy but for car camping it was nice! just thowing it out there that even a 40* rating on a bag can really be used way lower as an underquilt with little modification.
oh and making a gear hammock to go under you sleeping hammock is awesome! i loved being able to throw my extra gear aka bag down there and still keep it off the ground! plus my shoes too!