width of the base and other queries

[Moondog55]

Was in the Op-Shop yesterday and I found a cheap synthetic sleeping bag for $6- because it had a broken zipper. Now I have no intention of ever using this as a sleeping bag but I thought it might be useful to experiment with.
As a precursor to my next overbag project. I sleep on a 600mm wide mat.
I'm now thinking about the best width and design for the base of this overbag to maximise winter use and the wide mat.
So an insulated base? Or a mat sleeve or just tapes and buckles to secure the overbag to the mat?
All of these I have considered in the past but because I couldn't make up my mind I've not made the first prototype.
Was partly a money issue, at $6- money isn't an issue and the prototype will be usable but probably only going to add 5C as an overbag not the 15C I think is needed for winter and a LW down bag.
Bag has Dupont ThermoFill and has compacted to half it's original loft and now about 6mm thick
Thoughts?

[Moondog55]

Now having just cut off the zippers and laid it out flat I would not be able to use the finished overbag as the donor is a Regular sized unit but as a design process I can utilise it based on a Regular mat size of 480/500mm width. Flat the fabric is 1720 long and tapers from 1370 to 1170 and the cut off hood is 1370 * 300mm.
Pictures will be taken if I remember