Holes may be deformed due to warping, just drill them out.
Rubber:fill is ok, 50% max
Plastic:100% fill , use bridgeing and brim ,adjust support angle at 60 deg.
Use non-brittle filament for plastic-belt . Like HIPS.
Please report issues in discussion. I will fix.
You can cut the first few layers if cant get to sit flat.

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