Answer
Ventilation holes must be smaller than the spider can squeeze through. The smallest sling in the enclosure sets the maximum safe gap.
Placement
Use side and/or lid ventilation to create airflow across the enclosure instead of relying on one large opening.
Slings
Tiny slings can escape through surprisingly small gaps. Inspect drilled holes, lid seams and cable/hinge gaps before use.
Finish the holes
Remove sharp plastic burrs after drilling or melting holes. The spider should not meet a rough edge while climbing.
Airflow check
Persistent condensation and stale wet substrate can indicate too little airflow; extremely rapid drying can indicate excessive ventilation for a moisture-dependent species.