Aqueon has a series of "long" tanks. I recently purchased a 20 gallon long and turned it into a sump.
It is a simple sump, divided into a filter chamber with filter media and live rock below it. Then to a refugium and finally to a return chamber which houses my protein skimmer, heater, ATO float and return pump.
I had a local glass shop cut my baffling and used a tube of aquarium safe silicone from BRS to set my baffles. Some egg crate to hold filter media.
In all, I built the sump for around $100.
Things in the hobby are expensive enough. I go DIY when I can.
Dom