Just set this system up in early June. The display tank is on the right, out of view. It's at a 90 degree to this set up. Anyway, maybe it'll give you some ideas. That's a 45g on top and it's drilled on the bottom back right corner. A hose (can't really see it) runs from there to the float valve on the sump. I understand your sump is underneath your display. Maybe you could have a top-off tank beside your display and run a hose from that, through your stand, and then to your sump.
That float valve is a "Laguna". (Hagen brand, I think).
Hope this helps.
Sorry about that. Fuax pas fixed.
Cheers,