After modeling a full frame rail for my wrecker project, a stretch kit opposed to full frame rail would negate the need to model or print any lumps, bumps, rivets, holes.etc. If you use parametric modeling, you could offer on demand custom lengths without the need to make a new model each time.
I just finished modeling and printing these inserts earlier today. They fit great and I can't believe how much rigidity they added to the kit frame, even unglued. At 88mm long i was sure they would introduce a sag or floppiness but they did just the opposite. They also cured the bow in the kit parts. Printed in PLA+.
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Reason for stretch...