This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 竻 is uncertain. It visually combines 竹 (bamboo) and 力 (strength), but the original semantic or phonetic intent is unclear.
No clear meaning is available, so a mnemonic is not possible. Focus on recognizing the shape: bamboo 竹 over strength 力.
For ロク, imagine a lock made of bamboo that requires strength to open: lock -> ロク, and the bamboo 竹 over strength 力 shape reminds you of the lock.