I've looked into building my own tank several times, both for personal use, and for a science center I worked for. Unless you have the equipment and know how, it really was more cost effective to buy it ready made. There are just about every type of tank and company out there already, if you do some scrounging...you'll find what you need.
But, since you say it won't need to hold water...that's another story. It might still be worth purchasing a ready made one, but since the most troublesome and expensive aspects...water tightness and strength have been removed from the situation...
Yes, I've built enclosures that are not water tight. It can be really fun to plan and make it yourself, and then spend years enjoying the pets in it. I have a 4 foot by 2 foot enclosure I made for my bullfrog in my living room. Two solid sides, two plexiglass sides and a sunken 20 gallon tank for swimming. I built it on top of a coffee table, so the tank is visible beneath.
What type of project are you thinking of?