If you find yourself with back problems try a TRUMPLINE. This is AKA a burden strap in some period literature. It consists of a broad forehead band connected by two cords to your pack or item being portaged.
It's one of those native inventions that looks wierd nd actually works.

Fixed knives are like the old saying '4WD will get you stuck where you wouldn't be dumb enough to go in 2WD.'
In extremis a fixed blade is naturallly preferable. But for many good reasons, as the OP mentioned you can't carry one. If you can't do normal duties with a folder perhaps HOW they are being done needs rethinking.