Not sure of the question, but setting up a 20L is exactly what I did for a QT/frag tank. Drilled it for the drain and return. Feel free to click the link in my signature if you'd like.
If you're asking if you can simply drill a hole, install a bulkhead with a strainer (no overflow box), the answer is yes, but...
Keep in mind, just using a strainer will not provide you surface skimming, or at least very little. More importantly, the drain down during pump outage will be significant so you need to make sure you have plenty of room in the sump. Just food for thought if that is what you're thinking.