Well beleive it or not people were making tanks and keeping fish long before aquariums were manufactured. There is a nice video on this page of a member making a 50g. I have made a few small tanks with the biggest being around 70g. It is a simple process and the only reason people don't do it more often is the cost of glass or plexi.
I think if I had the skill, the guts, and the time then I'd give it a shot (well start with a small one first probably), but as I have none of the three not me
i made my own tank out of glass, first attempt lasted 1 week (bbbbiiiiigggg mess) , 2nd attempt a good friend helped me and it´s running it´s 3rd year its a 460 gal, measures are 3.3m x .75m x .70 m (130" X 29.5" x 27.6)