Thanks you all for the replies!
David, Richard and Alex, thanks for the baselayer suggestions. Fortunately the baselayers stay put, it's the midlayers and puffy that are problematic. I'll keep your suggestions in mind for the future.
Maybe I can find some kind of cycling fleece instead of my currently preferred Land's End ThermaCheck 100 half zip fleece.
Paul, if the interlocking layers are not long enough, the "locking" part is a problem. I am at the stage where I tuck the top baselayer in my underwear, it is an improvement.
Justin, wrapping the sleeping bag around my waist only covers limited situations. There is also the inconvenience factor of trying to wrap my waist with a 0 degree bag in the snow. Maybe I should be melting snow in the tent...
Doug, thanks for the Tyvek suit idea. It has limited insulative properties unfortunately.
Mitch, thanks, the minisuspender idea is something I didn't think of, but sparked an idea, see bellow.
Caleb, the more I think about it, the less I like the zipper idea. It seems too fiddly with the zipper being at the back. I am thinking about buttons and shock cord now.
I also got to thinking; if regular clothes don't fit me well, would a full body suit fit me? I bet not.
It is clear to me now that I have to make changes to my clothing:
1. Buy a cheap and long replacement for my Land’s End TC 100 fleece or add buttons (or some other type of fasteners) and elastic cord.
2. Buy some tall waist bibs instead of my M65 army pant liners, or perhaps sew some fleece above the waist. I think a high waist on thermal pants would be the most beneficial out of the 3.
3. Buy a long down parka. This is going to be expensive. Unfortunately these kinds of parkas also have hoods made to fit helmets (I hate big hoods and I don’t carry a helmet).