No roasting, and no plans to reveal that one of you is a bot...because I think you both might be...Erm, he would be more likely to roast you and then call me a bot lol.
Okay, short and sweet: this is not a difficult job. Build a box out of 3/4-inch plywood, complete with bracing around the top opening and at least one front-to-back crossbrace, using both glue and screws; cut out the window opening leaving a solid one-piece frame around the outside edge; waterproof the interior (liquid rubber, Pond Shield, there's lots of choices; my current favourite is Interseal) following the directions on the can in terms of mixing, drying times, etc.; use pure silicone to attach the glass panel to the inside of the front; drill for bulkheads; fill with water. Yeah, you can use fibreglass and add external frames and multiple cross-braces and all kinds of fancy crap...but at these sizes, none of that is even remotely necessary.
Do I get a few more words before you fall asleep? Okay, some disclaimers.
First...a certain minimum level of competence and experience with tools is useful. Do you have the skills and abilities to handle a large sheet of plywood and cut it into precisely sized pieces that are perfectly square at the corners? Don't expect to have that done at the lumberyard.
Second...a tank this size is not going to save you any money over buying an all-glass tank. IMHO, there are only two basic reasons to build a plywood tank this small. One is simply because you want to build one; perfectly valid justification, by the way. The other is because a tank built like this may leak if you are sloppy in construction, but it will never experience a catastrophic failure such as is possible with an all-glass tank. You will never have a seam give way and dump all the water on the floor in seconds. I trust a plywood tank way, way more than an all-glass one...and I spent a couple summers working for a guy who made all-glass tanks, and built a bunch while I was there.
Still awake? Okay, good! Now just do some research; there are tons of threads on MFK about exactly what you want to do. Read them, understand them...and then decide if you still want to do this. You're going to invest some time into this; the Dumbass Boogerman would have gone through three or four complete tank renos in the time it will take to get your new one up and running.
I see you're nodding off, so just one more comment; many of the threads about DIY tanks that failed seemed to have a common element. The guy builds the tank in his friend's garage, then gets four drunken buddies to load it into his truck, drives home on a bumpy gravel road, gets four more drunken buddies to slide the tank out of the truck bed, carry it around back, turn it sideways and on one end to get through the back door, down the hallway, around a sharp corner, down a rickety set of stairs to the basement, through a couple more doors and finally slam it down on top of the homemade stand that looks like it fell out of an airplane.
I build virtually all my tanks right there where they will live; the biggest trauma they typically suffer is being tipped forward gently onto their fronts to install the glass, and then back into the upright position. I have very few that ever developed leaks. Coincidence? I think not...
