I am over weight by about 20 lbs and also am just beginning so lack the strength and mobility to get squarely back onto my feet after rolling onto my back for the deck squat. An easy solution I found for this was to hold a 5 or 10 lb plate in my hands and this was enough to help me get back up. Of course this acts only as a progression to a perfect deck squat. Just wanted to share.
Cheers.