Let PETG 3D-printed parts cool down slowly on the bed of the printer to avoid warping

I've been printing quite a few registration plates to hold parts in place in production that need to sit as flat as possible on the production jigs.

I've noticed that taking the finished parts off the bed of the printer too soon causes them to warp: they come off the bed looking nicely set and straight. But as they finish cooling down unsupported, they slowly warp until they finally fully reach room temperature after 10 or 15 minutes.

So now I let them cool down slowly on the bed of the printer after the print is done. Ideally I let the bed cool down to room temperature: then the parts readily separate naturally as they shrink. If I'm in a hurry, I'll still wait until the parts release with very little force, around 35C - but no hotter than that, to avoid warping - as in this video.