If there is already a threaded bolt hole on the back of the speakers it shouldn't be too hard to fashion up an adapter plate with flat stock with holes drilled into it and various bolts/nuts/washers to go between it and that mount.
Or maybe the even 'easier' route might be just to use modify the end of that mount.
Unbolt the blue bolt so you are only dealing with the end piece.
Drill a hole at orange point, bolt through from the backside into the speaker.
Any of the above only requires a drill and some decent bits. No kind of real fabrication like cutting, bending, welding, making any threaded holes, etc.
Then if you want to get real fancy, or if the back of that was bigger than the back of your speakers you could cut it down a bit to make it look cleaner. Then get some flat back spray paint to cover it up again.
I like this route for what you are trying to do. TV mounts like that seems so cheap, it's probably worth for the little bit of work on your end. Even if you find something speaker specific it's going to cost 10x as much, or even more.
This reminded me of how much I miss my local hardware store back home; local family owned ACE going on 76 years now. Places like that are awesome for finding the right bolt, nut, washer, then only having to buy as many as you really need.