iShoot Metal Tripod Mount Ring Lens Collar Compatible with Sigma 18-35mm f/1.8 DC HSM Art, Lens Support Holder Bracket Bottom is Arca-Swiss Fit Quick Release Plate Feature

This is a nice precision-engineered accessory in an age of, well, not so precision-engineered accessories. It has to be to grab on to the lens like it does, but it's solid, doesn't tilt the lens, and can even fit (with a little lump on one side but room to spare) in the original Sigma carrying case while attached.