Make your own bracket

Rim Reader works with a bracket you make yourself if it does these seven things. Then run Bracket Calibration with your phone in it.

Seven things a bracket needs

Two straight edges, two prongs each. Each long edge has two prongs that rest on the rim’s lip. Ours are 196 mm apart.
Prongs that reach past the tire. The prongs stick out about 32 mm past the edge, so they reach the lip without touching the tire.
Phone flat, long side along the prongs. The phone lies flat, screen up, its long side along the line between the prongs and its top toward FRONT.
Phone held tight, bracket stiff. The phone is pushed into a corner and clamped, so it cannot shift or rock. Pressing on the far edge should not tilt the phone; ours is a solid 15 mm slab.
Flat back, steady on its end. It lies still on the floor, and stands steady on its end against a wall. The calibration and the floor readings need both.
A FRONT mark. FRONT points to the front of the car at every wheel. The edge on the phone’s right side is the rail edge; camber is read on it.
Run Bracket Calibration. It measures your bracket’s own small differences and refuses a phone that moves. Run it again for a new phone or a new bracket. Calibration, step by step

The rim’s lip is the reference. Its flat outer ring is square to the axle, so two contacts anywhere on it give a line parallel to the wheel.
Hard stops locate; the clamp only holds. A rigid rail, end stop and floor position the phone. Springy phone holders are too soft to hold an angle.
Contact on the prongs only, never the tire or the hub.
Calibrate, don’t machine. Any fixed difference either cancels between the two wheels of an axle or is measured once by the app and subtracted.