I was wanting to build an indoor pond for my arowana. The dimensions are 7' long by 4' wide. I was wanting to build it like the picture below. I will have a liner,a 2x4 band around the top, and have plywood screwed to the bottom as well. I was worried about it bowing but it will only be a little less than 18" tall. Does anyone have see any problems with this design? Thank you for your input.