It depends what you mean by simple and what you want to do with it. The simplest sump would just be a container that can hold water with a return pump. If you want chambers you can add dividers for all the reasons that were listed above. It is actually really simple to make a sump, even out of glass and silicone.
I'm not trying to hijack this thread or anything but how would you make a sump next your tank (I don't have access to the space under my tank). Also could I use a 10 g or 20 tall?