I've got an RX-100 on extended loan from my brother-in-law (who has my X-Pro1 on similar terms). Wow, this thing is small!
I wanted a better way to hang on to it, and didn't want to install a Franiec grip on someone else's camera. So, Step 1: Pull out some 1mm Op-Tech connectors, and attach my trusty Gordy strap. (There's a small cable clamp that hooks the Gordy split ring to the slot in the Op-Tech connector.) Ah, I feel better now!
But, I still needed a better way to hang on to it while shooting. So, off to the hardware store.
That's two copies of a quick 'n dirty grip; I mostly use the shorter one that has an Op-Tech connector on it, so I can move the Gordy strap from the end of the camera to the connector on the grip.
The grip consists of 1) a bolt, 2) a nylon bushing (the white part), and 3) a nylon washer (the black bit at the top). The bushing is meant for repairing screen doors, I think. Everything is held together with a bit of 5 minute epoxy.
The bolt is called an "elevator bolt". Who knows its intended purpose, but it has 1/4-20 threads, and the head is perfectly flat and about 1 inch in diameter.
The idea is that you screw the tip of the bolt into the tripod socket, attach the strap to the connector, slide the white part of the grip between the index and middle fingers of your left hand, as far as it will go, and bingo! A nice stable grasp of the RX-100. The head of the bolt is wide enough that there's no danger of losing your hold of the camera.
(Apologies for the quality of the photos!) Here's a better view of the camera ready for action in landscape mode:
For portrait mode, you can work either with your fingers pointed up, or pointed down, depending on what works best for you. Here's a fingers down view:
The bolts come in several lengths, so you can build a grip that matches the thickness of your fingers. The shorter one (about 0.75 inch) is perfect for my fingers.
The shorter grip cost me about $1.00; the taller one about $1.30.