S2S Traveller 2 as an overbag?

As part of my “Just in case I do get to visit a Canadian and Alaskan winter” set-up and because they were on sale locally I now own the Australian equivalent of the FF flicker.
I bought a Large.
I also have a Nunatak custom APEX overquilt which was to go with a sleeping bag I have sold on.
The custom APEX uses a mat sleeve as the base to suit a wide mat.
If I close up the new summer sleeping bags foot it is slightly too small for the footbox of my down bag and so I was contemplating making a very lightly insulated pad with a #3 zipper on each side and zipping the S2S Tr2 to the zippered and padded sleeve. My APEX quilt is 26″ wide, any ideas on the best width to make the padded sleeve? How much space will be too much?
My initial thought was to make it 20″, nominally the same as a standard pad which will still leave some space at the foot of the overquilt to stash my boots and perhaps water bottles. I have ordered a set of #3 coil zippers but they are still in transit so plenty of time to research and make a decision

yes it can be used that way if enough room inside for your inner bag.

Not when it’s closed up. It only fits over as a quilt; when I zipper it shut the foot is too small to allow full loft of the inner bag, hence my question about making a small base.
I should mention that I am a restless side sleeper.
The existing winter bag is a tulip shaped zipperless/wriggle in and the foot section is a parallelogram with approximately 12 inch sides