I read a good article a few months ago about creating large tanks from marine plywood. Only the front was glass. The real challenges were 10 making sure the glass back and sides were strong enough to support the pressure (I forgot how to do the calculations) and making sure there was a support structure in place strong enough to tote a couple thousand pounds (BTW this was a 900 gallon aquarium now that I think about it). Good luck.
I just met this guy in the aquarium club that just started in my area, heres his outdoor tank (I think it's 500 gallons with an underground well to heat and cool the water - we live in so. california desert, so in summer it gets really really hot).