Torso harness?

Have you looked at rock climbing gear?

https://www.mec.ca/en/product/5055-849/Technician-Harness

It seems you have looked at something like 'flying harnesses' and if so then I am not surprised they are a bit bulky for what you need because they will have all sorts of safety margins in their design to allow for dynamic forces (you may be amazed at what happens when a weight starts moving).
Maybe a company like this could a good place to call and start discussion? They usually love talking technical and problem solving.

http://www.bluechilliflying.com/harnesses

You could rig something rather lighter and less obtrusive such as webbing sling (check the link from Don) - these slings have breaking strains in the 100s kg so one static model will be no problem for a relatively thin webbing. In my climbing days, we practised creating an emergency climbing harness out of two webbing slings
Butt:

Shoulders:

And you can connect them together at the front with another small loop if you wanted. You could fashion something like this and connect at the back.

You could do the same sort of thing with them clipped at the back instead of the front. If you want to avoid loops round the thigh/groin, then you could look for climbing belts - like the ones Don posted but without the leg loops and the belt often has stitched loop that you could attach a karabiner to. If you can't find a belt then (as above) a sling with two strategically placed knots and slip a Karabiner through each knot to give 2 anchor points.
If the model has good core strength ad grippy shoes , maybe just the waist belt/sling?

I hope this all makes sense.

Don Haines said:

Have you looked at rock climbing gear?

https://www.mec.ca/en/product/5055-849/Technician-Harness

Click to expand...

Sounds like a good idea!