Actually 1 Ato "overpressure" will create a significant force on the acrylic lid .. You could visualize it like this -- on top of the lid place a 76 cm high column of mercury, with a diameter like the lid itself ...
It should be obvious that very significant stress is placed on the acrylic .. which is a brittle material .. not tough like Polycarbonate/Lexan
When you drill holes in it, or use a router to shape it, you will get tiny radiating cracks If you are not using VERY sharp cutters or drills and going VERY slowly in the machining. These cracks will act as fracture initiators.
The end result will most likely be a build up of stress on the lid, resulting in an abrupt and catastrophic release of all the forces acting on the lid .. Styrene vapours may well be accelerating the building of the cracks.
@finnluxury: Where in the lid did you drill the hole -- in the middle? (Where forces would be the greatest ..

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