Since Emmersed tanks are rarely about the "show" and more about growing healthy, plants, I would use smaller tanks, not bigger. More control, easier to light, etc, etc, etc.
I think you really underestimate how much an 8x2x2 tank would cost to even make yourself. There is more than one reason you don't see them all over..... Building a cheap stand for a tank that size would easily run a few hundred dollars. I think if just 2x2's, screws and glue it could easily run 120 bucks for just the stand.
The glass, easily well over 300 bucks if you do it right. 8' sheets of glass aren't cheap or easy to handle. I built a few 5 foot tanks and it was a nightmare. Building 10 gallon tanks is nothing in comparison.
If your goal is to pump out plants, a rack of 40 breeders is a better fit. Or other such height tanks. Easy to light and good size still. That or tubs. I use rubbermaid stock ponds for shrimp growout and they would work fine for plants.