The little clamp that could
This compact and versatile super clamp from Sunwayfoto, the CC-02 has found many uses in my kit. With multiple 1/4-20 and a couple 3/8-16 threaded holes it makes mounting various accessories very easy. It's solid aluminum and allows for quite a strong grip that's able to replace a mafer clamp in some cases. The male threaded inserts can moved around or removed entirely. Additionally, it offers Arri-style anti-twist for one of the female holes. Once I got one of these I quickly found myself buying 4 more to keep with my kit.
I have one that I keep a SmallRig Universal Power Bank Holder 2790 which is also great for power supplies and chargers as well. Another has a cine-lock style QR.
