I have been using a Fluval M100 since I set up my 20long in May. It's rated for 30 gallons.
I run my filter (Fluval C3) and the heater on the short side of the tank, as opposed to the back, so the filter pushes water long ways. It doesn't take away all the 'clutter' but keeps the back wall nice and clean.
There's nothing that I don't like about it. I like the reflective finish..blue light..adjustable settings. Very consistent temps from what I've seen. No difference in temps from one side to the other. Measured with Traceable therm.
I've seen it for about $20 everywhere. PetSmart/Co, local stores, and Amazon.