I've been playing around with the Peapod method of Hammock insulation for about 8 years now.
I have found it to be the easiest and simplest method to stay warm in a hammock.
I've showing learners how to use it to keep warm in a hammock in temps to
-15*F with great results. They where able to sleep warm and comfortable for 8 hours without adjusting the system.

The basic hammock thaty works best is your basic tied end paracloth hammock 60" to 65" wide by 108" to 120" long.
A ridge line can be used but not nessery . If used should be detachable on one end.
You can use a hammock with bug net if it is the type that can be flipped over as the Exped combo.

The use of a reflective foam windshield garde I find gives body to the hammock that helps the user to get in and center one self in the hammock.
Adds to the lower insulation, vapor barrior to your lower insulation and gives you a quiker warm up time when getting into a cold sleep system.
I don't leave home without it.

The foot end is tied to the end of hammock with the cinch cord of the draw string bottom after tighted around hammock.The head end is from a loop on the hood.
Add cloth string pulls to zippers or contact to metal zippers with bare fingers can cause froset nip in -10*F and below temps !

I find that that a sleeping bag with a draw string bottom and center zipper
works best, that is extra long and wide.
It allows for easy in and out of the system and the draw string bottom allows the user to seal the bottom of the system to the hammock.

The bags listed bellow are ones I have used with good results.

-Feathered Friends Winter Wren Long Down ( 2 lbs)
Great bag, a little confining , only comes in standerd width (60" girth), combined with the Hilleberg Bivarack and simple paracloth hammock
I have slept many a night warm and comfortable down to 20*F with my clothing and a foam reflective windshield garde.
It was my first modern hammock camping sleep system.

-Exped Dreamwalker LG 450 and 650 Down ( 3 lbs)
Great bags and versatile, by combing to 650 bags with windshield garde and a hammock sock I was able to sleep art 22-*F warm and comfortable.
With these a person can go down to 30*F then added clothing or a second bag if you go colder.
With two I think you could easly go to -30*F with windshield garde and clothing.The best of the off the shelf unit yet for the DIY Peapod I found yet.

-Wiggys Ecotat 20*F bag ( 4,5 lbs)
Great value for the dollar, combined with clothing, windshield garde and an added foot/lower leg bag have gone down to -14*F.
A tough and durable bag.

I have tried and used others. But have found that the draw string bottom and center zip works best.
I think if you added a draw string bottom with one that had a zipper opening you would be ok.
I just find the zipper openingjust does not seal as well. Side zipper is harder to use when comparied to center zip.

-warmer, closed system. No gaps in insulation as one shifts in the night.
-multi-use systems, (can use as sleeping bag, over garment and quilt)
-easy to add layers for lower temp ratings.
-less fiddle factor ( your setup, your in, your out)

-More confining then with under/ over quilt system.
-Can be claustorphopic to some.
-Only allows for a partial asym lay.

I hope this helps and good luck making your own system !

Always perfect your skills and equipment before you go out in the deep cold ( -0F and below).
Also go with an experenced person for the first time.
In those temps a person can be damaged quickly and have to live with that damage for a life time .
Your resposiple for your own safety !!

Be safe and have fun outside !