Use had better (negative: had better not) and the verb in brackets to complete the urgent warning: The wind is already lifting the tarpaulin. We ___ it before the next gust. (secure)
Bonne réponse
had better secure
Explication
The requested had-better construction takes the bare verb secure and conveys a warning prompted by the next gust. Must secure may also express urgent necessity, but it does not follow the frame explicitly required here.