well, I have only used the commercial version of a Speer Peapod, so I can't give much help when it comes to using a sleeping bag for that purpose. As Shug and several of the others have done so successfully.
But the first thing that comes to mind is: it seems like you're going to need some sort of suspension, even if it is only some nylon cord on the bags/pods ends, such as is used on the Speer Peapod. In other words, if I let my Peapod just hang however it seemed to fall without tightening up the nylon cords on the end, I would also have a rather huge gap underneath me. But by using the nylon cords to pull the ends of the Peapod closer to the ends of the hammock, I end up with about the appropriate fit. Plus these cords allow me to cinch it all down on the ends, "sealing" the ends closed. Now mind you, there is - before I get in- still always some gap(6"?) under the hammock – left there on purpose. Because when I get in, the hammock will sink (sag) a good bit more than the pod which is tied to the hammock ends. So what I'm shooting for is that once the hammock sags all that is going to, I end up just about in contact with the inside of the Peapod on the bottom. If there is still too much of a gap, I have to get out and tighten up the Peapod some more – in other words pulling the pods end closer to the end of the hammock. But if it is too tight and I am compressing some of the loft, then I have to loosen it up to give it a little bit more.
I can't help you with the hanging under the shoulder problems. The Peapod is tapered on both ends and completely surrounds the hammock from one end to the other. So if you close the Peapod's Velcro all the way, then the hammock is 100% completely enclosed. A sleeping bag that has a hood would obviously work somewhat different. But hang in there! Assuming your sleeping bag is big enough to start with, you should be able to work it out. Several people have and have been rewarded with some extremely warm sleeps. If you haven't already, watch Shug's videos.
For I consider that the sufferings of this present time are not worthy to be compared with the glory that is to be revealed to us....that the creation itself also will be set free from its slavery to corruption into the freedom of the glory of the children of God. For we know that the whole creation groans and suffers the pains of childbirth together until now.