It is a bit tough to hang 2 loose boxes ... that is one reason I went with bending acrylics.
The one solution I saw on the web was to drill the boxes & use a few zip ties to hold the two boxes together, then hang it on the tank. That looks a bit loose on the pic, but the guy says it works. A better way I think is to use a hanger. If you can find a U-shape piece of plastic with ~ 1" gap .... may be a long & narrow candy box or something that you can cut up ... you can use that as a hanger. You can bolt the boxes to the hanger with nylon bolts. You can also bolt the hanger to the tank if you drill a hole ( or 2 ) on the tank side of the hanger, & run a bolt through it so you wedge the hanger tightly onto the tank.
Drilling a tank is always risky, even for a pro. Also, most newer tanks are tempered, and the risk of shattering the glass is even higher.
A good store bought overflow is this:
Reef Aquarium Filtration: Eshopps Overflow Boxes
Quite a few here have this & have good things to say.
I would avoid the continuous siphon overflows (no siphon tube, wier type). Because of the low driving pressure of the design, the siphon is prone to air trapping & losing siphon. There were many problems with that reported on the various forums. If you use a continuous siphon OF, you need to run it with a small PH or venturi pump to continuously evacuate out the air from the siphon. Problem with this is that a lot of the "CPR" style OF + pump kits on Ebay & other sites have pumps that are cheap & prone to break down .... and there have been reports of floods when one of those pumps stop working ....
After doing a lot of reading on the pros & cons of the 2 HOB
OF types, I decided on a siphon tube design with a high driving pressure. So far ... touch wood ... no floods ... running for 2+ years. <This is basically the eshopps design ... and people here said they never have siphon breaks.>