Looking for economical retrofit overflow box

[Reefs and Geeks]

I am looking for an economical way to add an overflow box to my current overflow. My tank is drilled near the top with (2) 1" bulkheads, and I currently have them both connected by PVC fittings, and a long slit cut across a peice of PVC connecting the two bulkheads to act as an overflow. This has been working suprisingly well for about a year now, but I'm looking to convert to a herbie style overflow and also converting to a basement sump at the same time. I found a retrofit overflow box on BRS, but it's almost $80 for the size I'd like. Has anyone found a cheaper option, or perhaps a decent DIY option to add an overflow box to a tank that is already drilled? Link below is what I found. It's the CPR retrofit overflow box, large size.

https://www.bulkreefsupply.com/cpr-retrofit-overflow-box.html