IYHO who makes the better overflow box for 20g

[reefling]

+1 to drill. Much more fail safe. I would make 2 overflows incase 1 gets blocked. 3/4" is probably fine. You could have the glass cut at lowes for a bean animal and the cost would be like 12$. The biggest expense would be 3 bulkhead fittings for 7$ea.