Right Corner Overflow Box
Pricing starts at $65.
This is the silicone I recommend, found near the glues in the paint department at Home Depot for about $5. You don't need a caulk gun nor a huge tube of silcone for this project. :)
Using Aquarium-safe silicone found at your LFS or Home Depot, glue this overflow into the left corner of your clean dry tank. Let cure for 24-48 hours before water testing.