If it's an acrylic tank, just drill it with a high speed drill and the right bit.
I've done it without any problems. Then, just build an overflow for the corner. The trick is ensuring you don't make the "teeth" of the overflow to wide, or too deep - otherwise you'll:
Too wide = pull fish into it
Too Deep = your tank will never be topped off / water won't reach the top...unless you have a powerful pump...
Best,
Stieger