ッ is usually transliterated with a double letter, thus your example リュック would be transliterated as ryukku, whereas your other example would be ryuuku. To me, speaking a long consonant is like starting to make the consonant sound, pausing for a short moment, and then saying the full syllable.
I would say リュ and immediately after, but before I completely stop airflow, put my tongue in the place where I will make the next sound, cut airflow (just for a moment), and then make the next sound.