People tend to use Aquaclear 110's and with a simple modification (baffle) can be made into a refugium where macroalgaes can grow or whatever you plan to use the space for. Two 50w heaters (Visitherm or Tronic) fit vertically as well, although, you'd really only need a single heater. If you just want to diy for the heck of it, then I would head over to nano-reef.com where I'm sure the diy section has many refugium threads.