My favorite putting mirror ever, now in collaboration with Always Press. Stainless steel double sided mirror with a shoulder mirror/ruler, 2 marbles for gate drills, and a CUSTOM "bet on you" sleeve to carry it in.  The sleeve makes it easy to fit in your bag so you Always have it on you. 

From https://shortgamegains.com : 

The Steel Trigon Mirror is a double-sided reflective plate for checking eye line, practicing start line, face alignment, aim, and enforcing center contact. It also acts as a stencil, working alongside the Putting Chalk Pens to create an identical alignment setup on grass. Next up, the Trigon Toolkit includes a double sided, reflective steel shoulder alignment mirror. The flip side acts as a ruler to measure setup and calibrate your stroke length.