I like the idea of "right as rain" meaning something true or plumb. Rain being something that falls in a straight line from our earthbound perspective. Maybe from "wright" as in shipwright or wheelwright, one who constructs or repairs things, one who works with precise measurements.

Right as a trivet? Something sound or stable? I believe most early trivets were set on three legs, like a milking stool, as an object supported by three points can't wobble.

Right as ninepence? Something unchangeable or secure. Ninepence is ninepence.

Just some thoughts.