Corner 'floating' overflow
For smaller systems, a little overflow box in the upper right or upper left rear corner may be the best option if the tank can't be drilled through the bottom pane of the aquarium.
Held in place with clear silicone, available at Home Depot in the GLUES section.
Pictured: 6" x 3.25" x 5" (internally 5.75" x 3" with 1" teeth) for rear RIGHT CORNER.
Specifiy LEFT REAR CORNER or RIGHT REAR CORNER
Custom sizes available upon request.