Is it possible to reinforce a cedar planter with pond liner in it to hold 40 gallons of water?
How would you go about building a rectangular planter to be a water garden?
I would start with 2 x 12" 8 feet long. Use metal brackets and screws to hold them together. Put the liner in the box. Use a 1" x 2" top piece of lumber to hold the top in place. Just nail them in, but NOT until you have filled it up with water. Don't worry about wrinkles. They disappear from sight after you get plants in there.
If you are using an existing box, do the same thing. Be sure to measure the bottom and both sides, then add a foot of liner so you have enough of it.