Placing the wedge pillow between the boxspring and top mattress should definitely help with stability and comfort. This way, your still lying on the top mattress surface, sheets and all.
The Mattress Genie wedge lift may help - it's an inflatable wedge that sits between the boxsprin and mattress and inflates to the proper position when you need it. Because it's adjustable, it's easier to find the right level of elevation for your situation. Here is some additional information on the Genie wedge lift.
Also, some additional sources:
Jackson / Seigelbaum Gastroenterology
STS Patient information
Thanks for that info...well, my primary doctor said that it shouldn't be placed between the mattress and the box spring.
so, honestly, this is all very confusing to me that my doctors don't seem to say the same things i'm hearing on these posts (about anything..sigh...)...