I got the inspiration for this build from uarujoey on youtube. He has a series of videos where he demonstrates building a 6 x 6 ft tank with a plywood bottom.
This tank is a combination of recycled glass from my old tank with a new front and sides. I used a tile saw to cut up the glass from my old tank to make the corner overflow and the euro braces. The bottom of this one is 3/4 in ply with 1.5 x 0.75 inch strips around the perimeter. The perimeter strips are on top of the plywood and are glued and screwed from below. I attached the front and two sides but left the rear strip unattached to be screwed on after the glass panels were in place. I then drilled the bottom for 4 bulkheads (3 drains and the return). I sealed the plywood with black Pond Armor epoxy. This stuff is very thick and difficult to mix and doesn't go on with a brush, you have to use a Bondo spreader. Joey gives a pretty good description of working with Pond Armor in his videos. After the epoxy cured I sanded the seam areas where the silicone would adhere as per the instructions. The rest of it went together the same way as any other glass tank. I used Momentive RTV103 black silicone from Grainger for all seams.