Late to the party but i have spent way too much time rigging these out. Real quick though, what lenses are you planning to take to Japan? If you are using heavy glass, the grip requirements change quite a bit for the fp. For ergonomics on your budget, the SmallRig Cage for Sigma fp and fp L 2271 is a solid foundation. It adds some much-needed weight and surface area to actually grab. You might also look at the Sigma HG-11 Hand Grip if you want to keep it minimal, but the cage is more versatile for adding bits later. For power, that 1200mAh BP-51 is basically a joke. I suggest:

• Wasabi Power BP-51 Battery 2-Pack and Charger
• Anker 313 Power Bank PowerCore 10K 10000mAh Carrying 4 or 5 spares is usually the most efficient way to handle a full day of street shooting without a bulky V-mount setup or cables everywhere.

@Reply #1 - good point about the glass, but honestly, I gotta disagree on the cage idea for a walking trip. If you are hitting the streets in Japan, you want to stay light and discrete! I love the Sigma HG-11 Hand Grip way more than a bulky cage tho. It is fantastic because it adds just enough depth for your fingers without ruining the aesthetic. It makes the fp feel like a real tool rather than a slippery soap bar. Since your hands are hurting, you absolutely need to check out the Sigma LVF-11 LCD Viewfinder. I know, it looks massive. But it is an amazing piece of gear for ergonomics! By bracing the camera against your eye, you get way more stability and it takes the strain off your wrists. Plus, using a screen in bright daylight is a nightmare, so this thing is basically essential for travel. Regarding the power, dont bother with external power banks for street photography. Just grab a few of the Powerextra BP-DC15 1600mAh Replacement Battery packs. They are super affordable and honestly work just as well as the originals in my experience. You could buy a handful and still stay way under your $300 limit. Just swap them out whenever you stop for ramen! It is a much more practical way to handle the battery life without adding a ton of wires and weight...

Just found this thread. Honestly the fp is such a letdown out of the box... I had issues with it for months before I finally got it usable. The power management is just not as good as expected for a travel camera, especially for street stuff. Before I give you my full rig list though, what lenses are you actually taking to Japan? If you are using something heavy it changes my advice on the grip situation completely. Quick tip for the power issue... skip the tiny internal batteries and grab a Zitay Sigma fp D-Tap to BP-51 Dummy Battery to run off a small power bank in your pocket. Its way more reliable for a full day of shooting than juggling six spares. For the hand cramps, try the SmallRig Baseplate for Sigma fp APU2671 instead of a side grip. It adds just enough height for your pinky to rest on without making the camera look like a brick. Really want to help you get this sorted for your trip tho because the sensor is actually incredible.

I had the exact same issue when I first grabbed the fp for a trip last year. It was kinda like holding a wet bar of soap and I was honestly worried about it slipping... I ended up finding this simple third-party wooden grip that screws into the side. It was super cheap compared to the official stuff but made the camera actually usable for more than five minutes. For power, I quickly realized that swapping batteries every hour is a nightmare. I just started keeping a small external power bank in my pocket with a long cable. Whenever I stopped for coffee or was just walking, I would let it top off. It made the whole trip way less stressful because I stopped checking the percentage every two seconds... I'm really happy with how it turned out because I didnt have to spend a fortune. TL;DR: A basic third-party grip and a pocket power bank. This combo fixes the handling and power issues without blowing your budget.