I've made two sumps with 20Ls. They work pretty well, especially for the money.
I like using the three chamber design. In this order:
Left Middle Right
DSB/Fuge Skimmer/heater Return Pump
Feed the center directly from the main tank then use a MJ400 or 600 to feed the DSB. The flow will then flow back through the skimmer area and into the return, along with all the extra directly from the center.
I have all of my flow going through my downdraft skimmer now. I like this alot. I get lots of nasty's and everything is filter everytime it goes down the pipe.
Aquaphile has one like this (used to be mine) and he is doing it like this. Ask him and see if he has any suggestions or improvments.
Steve