I know a custom door leaf will scale to fit the door frame you use it in - no matter what size you model it.
If the frame object does not have an option for tolerances, then maybe you could model an extra air space slab (invisible/transparent material) with no perimeter lines around your door slab before you save it as a custom door.
Effectively you are modelling the tolerance.
I haven't tried it but it might work.
It won't be adjustable but should create a small gap if it does work.
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