I printed a lid for my refugium which is about 10" wide (wider than my printer can print) - had to print the lid in sections that slid together. The lid fits my tank and hold my lights perfect, and is customized for the space under the main display. The cool thing is, that once you make it, you realize other modifications you want to incorporate, so it becomes a fun customization process. Probably not cost effective, though...
I use Tinkercad for designing (free online), and have Dremel 3D printer, so I use their DigiLab 3D slicer (free download).