Jason, the thought of first finding then hacking every car is daunting to me, mating edges, etc.
After experiencing all of the print length issues at SW, I had been considering designing a sliding tongue concept into an underframe of an RP core to allow both halves to adjust to the length of the etches. I have successfully used sliding features on other designs by allowing a 0.065mm clearance on all surfaces that will interface (technically this gap can be printed in place, but I would not). Some holes above the tongue in the mating pocket will allow for capillary gluing to secure the final length from the inside of the car. Any resulting gap in the core will be hidden by the sides, and only visible underneath.
Just a thought.
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