Fitting shelf between 2 wall units


Hi,
I have 2 box wall units approx 5ft long by 1 foot high, with a gap of about 18 inches in between, roughly 5 feet from the floor. I want to put a shelf between them, what's the best way to attach it between the ends of the boxes ?
A wall unit similar to the 2 I have is here:
http://tinyurl.com/chceg8
It's concrete wall, is it worth using the "hidden" wall-mount style and not attach it to the existing boxes, or can I attach it to the 2 ends of the boxes with a mount that doesn't look too "amateur DIY"-ish? My main worry is making it safe enough to hold a stereo system, so my usual "no more nails" and blu-tack isn't an option here. I have a big noisy hammer drill but a finite amount of skill and patience for these jobs :)
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Look how the shelf is held in the units. It probably has those little "L" shaped metal plug brackets that just push into a hole in the upright. You can buy those at a DIY store, a little careful marking and drilling is all that then needed. Wrap a bit of tape around the drill bit to stop it going too deep, or alternately if the shelf is going to line up with one in the unit just drill right through using the existing holes as guides.
Mike
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