I found Plastic-Mart while searching for a suitable water tank for the trailer. They have this awesome table that lists all their tanks by volume and includes length, width, and height dimensions. There's tons of options, and most given volumes have a few different shapes available. When you order a tank, you can give instructions on what size and quantity of fittings to put on the tank and where, which is great. We requested a typical feed, drain, and vent setup. I was a little worried until the tank arrived because we'd laid out the frame to exactly the dimensions given in the table. I had no idea who these people were, or if their drawings were to be trusted. And there's no tolerance given on the dimensions either. What if it comes in 1/8" oversize? I'd have to cut the frame apart, which I really didn't want to have to do. We were ecstatic to find that the tank fit perfectly. It was built to exactly the dimensions given.
I'd held off on making any mounting components until we actually had the tank so I could use it as a reference for sizing things. Once we had it, I got to work making up a mount for it. It's pretty simple, really. Two of the frame crossmembers sandwich it to prevent fore-aft movement, as you see in the picture. On the left and right, two lengths of steel angle between the frame crossmembers prevent side-to-side movement. A simple steel angle frame lays on the top and fastens to the lower mounting to prevent up-and-down movement. A representative at Plastic-Mart told me that the entire bottom of the tank has to be supported. So I cut a skid plate for the water tank and welded it in place after making some small drainage holes. We don't want any water hanging out near our water.