On a semi-serious note, a sheet of plywood, or a 2'X8' rip, with 2X12 sides and ends, lined with plastic, would make a shallow test tank, cheap. If you have it outside, you can just put it together, paint it, and no plastic. As long as you make good cuts and screw the pieces together, it will hold water for testing, and, eventually, self drain. That might eliminate the problem of trying to drain it.