A tunnel is a trickier project than you would think. I've been involved with installing and keeping them up. They drastically change how you can move water in a tank, not to mention are a PAIN to clean. The tunnel also distorts the fish as you look at them. In fact (opposite of popular belief), the tunnel will SHRINK the animals by about 20%.
The flat (or even a curved) window is by FAR easier to install and keep clean.
However, if you have enough $$, you can do it. Getting one made and shipped it the hard part. The curve of the tunnel makes them hard to move. Plus, you'd have to look into what you really want...one seemless part or severla sections. The sectioned method is easier to move and cheaper, but then you have to have someone put it together once it's on site.