Either way you still need to buy materials for the sump and plumbing, media to fill it, and a return pump. if you go with a DIY style sump (using a rubbermaid container), you're only looking at an extra $10.
But I understand trying to mke use of what you have.
You may be able to get away with the 20L, but it will be cutting it pretty close. the reason for the larger size is to accommodate the water that will backfill into your sump when the pump is turned off. Depending on the operating water level in your sump, and how much water backfills into it, you could be looking at a soggy floor if your sump tank is not big enough.
Personally if suggest sticking to the 25-30% "rule". But I'm sure there's a way to use your 20L if the proper precautions are taken.