Basically, I drilled a few extra holes on each side of the bottom of the middle shelf (which is really the bottom shelf of the second unit). The goal is to attach the middle shelf to the first set of side supports in the same way as the top shelf attaches to its side supports. You want to drill your holes on the underside of the middle shelf to mirror the holes on the underside of the top shelf, since it is attaching to the side supports in the same way. These holes aren't present in the middle shelf to begin with because, since it's really the bottom of the second unit, the only thing normally attaching to its underside is the four legs.
This makes the whole thing fit together beautifully, because you're able to use the standard screw-locks it comes with to put it all together as if it was designed that way in the first place. I used no glue -- only what came in the two boxes -- and the thing is as solid as a rock.
Once you start assembling them it will become quite obvious. The only place you need to drill is on each end of the underside of the middle shelf. At the end of the project you'll wind up with an extra top shelf and four extra legs. Since the Lack units are only $70, this looked a lot better to me than any other $140 TV stand I was considering. It's very solid and substantial looking yet sleek at the same time.
Here's my only caution to people considering this setup: the space between the shelves is not high enough to accommodate most receivers. I had a fairly standard-sized old Panasonic receiver, and after I had built this TV stand and wired everything up I went to slide the receiver in and it was 0.25" too tall! Fortunately, I was going to buy a new receiver anyway so I purchased this more compact class D receiver and it fits fine, as you can see in the pictures. Sounds great too. But most receivers are a little too tall to fit.
Good luck!
Sweet -- thanks, man. I'll post up once I get it together.
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