I apologise. It can be a problem when one is writing in a language in which one does not think…to use the term that means the opposite of what one intended. Your question made me realise the error of my choice of words.
The tabernacle must be opaque; it must be solid and of material that one cannot see through. I wrote “non-opaque” which is the wrong word for saying that it is forbidden for the tabernacle to be transparent or in any way semi-transparent or even “translucent”.
Canon Law states, in English translation:
*Can. 938 §1. The Most Holy Eucharist is to be reserved habitually in only one tabernacle of a church or oratory.
§2. The tabernacle in which the Most Holy Eucharist is reserved is to be situated in some part of the church or oratory which is distinguished, conspicuous, beautifully decorated, and suitable for prayer.
§3. The tabernacle in which the Most Holy Eucharist is reserved habitually is to be immovable, made of solid and opaque material*, and locked in such a way that the danger of profanation is avoided as much as possible.
§4. For a grave cause, it is permitted to reserve the Most Holy Eucharist in some other fitting and more secure place, especially at night.
§5. The person responsible for the church or oratory is to take care that the key of the tabernacle in which the Most Holy Eucharist is reserved is safeguarded most diligently.
Inaestimabile donum states:
25. The tabernacle should be solid, unbreakable, and not transparent
“Non transparent” was the concept I was grasping for in English when I wrote “non-opaque.”