I'm looking at getting a Baader Vari-lock to fine-tune backfocus between a sensor and field flattener. How robust are these spacers to weight and shifts in weight? I worry that the nature of these things will allow a little bit of tilt between the optical axis and the sensor plane. I'll be imaging at f/3.5-f/5, so tilt would be a concern at these focal ratios. I'll also be using either a QSI-690-wsg8 or a full frame DSLR.