With or without the washboard, Clairebear is steering you correctly with the harness idea. Something like:

Picture a belt at above-waist level, now add wide-ish strips over each shoulder in the manner of suspenders (but non-elastic). Next, add another belt at hip level, connected to the first one by several vertical strips (I'd try 6: 2 front, 2 back, 1 at each side-seam). The two front strips each have built into them the equivalent of an extra long pen-pocket. Think of the skinny, 1-pen type of slot you see in briefcases, or at one side of some shirt pockets, but longer. Into these pockets you can slide an upside-down U-shaped frame that has clamps for harmonica et al.

Use sheet foam or packed layers of quilt batting on the pen-pockets to protect both your guitar and yourself. Line the bottom of these pockets with heavy leather or canvas scraps, or some kind of reinforcement. Make the harness fit snugly enough that the U-frame won't wobble too much. A 3rd vertical might be appropriate to help resolve this, or a cross-brace in the U-frame.

I'd use scrap fabric and safety pins to rough out such a harness, then when all is figured out, use it for a pattern when making the real thing out of old belts, lawn-chair webbing, whatever I could find. (Line anything plastic or nylon with terrycloth to allow air circulation.) Finalize the distance between the two belts such that the "pen-pockets" are long enough to give good support to the U-frameShape the verticals of the U-frame so that the blown stuff when clamped on is the right distance from your mouth.

I picture the U-frame might be made of the kind of metal that is used for sleeve-type election signs to slid over them. Maybe it would be too thick or heavy, but maybe not.

Anyway that's what I'd try. If I had a kidney-belt like they use at the local Staples, I'd look at that, maybe even use it.