Based on your measurements, the 3 inch PVC will be a little harder to come by...but I don't think you ever specified that it could be round...and a 6 ft plus piece of 3" PVC filled will be a little hard to manage..or did you mean 7.5 inches instead of 75?

So what you are looking for is a square container that is 3 inches wide, 6 ft+ long and 1 to 3 inches deep?