Hello Anna,
It’s been a few month since you posted this, and I was wondering if you had found a solution? I am renting the same harp, only in double-strung.
The only thing I’ve found is not a strap or harness, and you wouldn’t be able to play this FH26 standing up. However, it might change the way you sit down. I’m talking about the harpsicle stick featured in Laurie Riley’s video about double-strung harps (about min 7 on the Youtube video).
This is what she’s talking about :

And you can find more infos here : Harpsicle Stick – Harpsicle-Harp.com
Hope that can help! But if you find the strap solution, I’m definitely interested in hearing about that.