If you like using Anti Cant Devices, here is one that puts all of the other Anti Cant Devices to shame.
Anti-Cant Device (bubble level, ring type)
This device is available in 30mm and soon will be available in 35 and 40mm diameters.
It can be positioned anywhere on the scope tube (can slide back and forth), for maximum visual use.
It can be made in any color to match the scope.
It can also be rotated to the angle you wish. Usually the bubble is aligned horizontally to be at a 90-degree angle to the vertical crosshairs in the scope. A 0 degree angle to the horizontal line or stadia is also a good way of doing it in the absence of a vertical hair or stadia line.
Note: it is not important to have the bubble exactly aligned at a precise angle with the horizontal or vertical elements of the scope (Reticle, turret, rail top etc.), as long as the bubble is placed in the same position and alignment each time you shoot. It is a common mistake to assume that the accuracy of the repeatability of the device depends on the exact vertical or horizontal alignment, but it is very important to make sure the location of the bubble is repeated each and every time you take a shot.