The STF-8 does not come with an adapter ring for the Olympus 90mm macro (effective 180mm.) I have come up with one solution. The flash heads can mount on 1/4 inch threads. The Platypod goosenecks have those. The base is a Platypod Max. It can be mounted on a tripod as it has a threaded hole, or you can add a release plate.
This might even be better than a flash ring as you can position the heads in many more positions. A pair of tiny Smallrig ball heads gives great flexibility in light positions.
One improvement might be a long mount on the lens for back and forth movement.
Led lights or other flash units will work well also. I will try my cube Led lights soon.