What's the adjective for lockboxes? Here's the word you're looking for.
Included below are past participle and present participle forms for the
verbs lock and lockpick which may be used as adjectives within certain contexts.
“Within a period of two or three weeks, it appeared that the main building was no longer lockfast and was being regularly entered by unauthorised persons.”
“It was such a scene of confusion as you can hardly fancy.164 All the lockfast places had been broken open in quest of the chart.”
“All the lockfast places had been broken open in quest of the chart.”