there are so many ways to make one...
what i envision for a smaller tank would be a large initial section for whatever you might want to have in there, skimmer, heater, refugium, etc.
then i would put some sort of eggcrate shelf in there to put the skimmer on and would keep it as close to the sump inlet as possible (that is if microbubbles are a huge issue with you).
and if microbubbles are just that, i would put in a baffle or two just before the outlet or submersible pump.
keep in mind that the last section will dictate how much water you can have evaporated before the pump starts sucking air, so if you are not going to have a top off system you might want this to be of a decent volume.