I have the same thing with my GH overflow. I was told you have to drill a hole into the 90 at the bulkhead and put some tubing in it. Haven't tried it yet though.
Yes sir he built it, glued it, rodded it, and shipped it . Extra $25, but so worth it. All I had to do was send him photos of setups I liked and he did the hard work.
Ahh so I can at least let it mix my salt for me so thats a plus