1. Go to Canal Plastics, buy 4 pieces in black to make two L shaped in either corner. 2. Rig up your drill so it's situated like a table saw, drill the teeth.
3. Glue the pieces of acrylic together to form the L shape, let cure overnight.
4. Silicone them in place (the weight of the water should be plenty to make sure they stay put.
DIY corner overflows for muuuuch less than what you'd pay to buy them or have them made = more cash for coral!
I'll see if I can find a link to someone who has done this, I'm sure it's been done.
I've done it and it's easy.
I used a single piece and bent it with a heat gun-no seams!
I then filed the edges at a 45 degree angle
And glued on gutter guard at the top to as a strainer.
No teeth-better skimming!