Beetroot
Lil-Rokslider
At $135 I don't see a reason to buy the Halo X.Any preferred Flatline Ops models? I remember Frank recommending the Halo X ...
The Vortex Pro or MKM machining bubble level is the same shape and layout (meaning it sits just above the parallax turret and easy to see) for 1/2 or 1/3 the cost.
I've got a few of the Vortex Pro bubble levels and like them.
I tend to agree with Frank, use them as a training aid so you train yourself to not need it in the field.
Saying that shooting both eyes open makes it easy to see, but you don't want to be spending all your focus fine tuning the bubble level.