I've build my own big plywood tank a while ago. It might seem overwhelming but definitely worth it if you have the right tools, a plan and patience. It is not necessarily cheaper if you build it properly. But you can make them any size you want. I wanted more length than height so I custom build a 8 foot tank for myself.
I made it with epoxy resin, fiberglass cloth and epoxy paint. Allot of work but I love my tank. And so do my small clownfish in a 8 foot tank.