Just to jump in here. All of my Hydroponics reservoirs are walmart specials. The only one that flexes is the 45 gallon container. And that only happens at the top. I have two underbed storage containers as small reservoirs and were filled the top with solution and did not flex. All you have to worry about is them drying out and cracking under large wattage HID lamps. Not a common practice in darkroom work so that means no worries at all.
To add to what titrisol said. Instead of all the cutting and fitting of thereads that will leak just drop a hoe from the top to the bottom and cut a drain hole on the other side. Easy as sin and has worked to flush hydroponics reservoirs for years.
Just my 2 cents.