To fill the gap between the tubes, a .060 x .060 strip was glued in, followed by two .030 x.030 strips.
Some slightly over-size strips of .040 styrene were glued on the ends. (Love those brass clamps.)
Using my smallest chisel blade, I removed the peaks of the three strips, leaving a flat surface.
With files and sandpaper, the ends are brought flush with the tubes. It is a little hard to see, but I glued a .010 x .125 strip over the cleared space in the above photo.
These two stips will form the concrete base of the tank. This step is easier if both strips are filed as one unit. Take the way oversizeed strips and glue them together with small dabs of gel glue. Don't use liquid cement as it will wick all the way through the joint, bonding it completely.
A few minutes with knife and file and you have a custom-fit base. Notice I'm using Squadron's Green Putty to fill in the sides of the tank. On the original tank I wrapped the sides with thin styrene. It was difficult to do, so I opted for the putty.
Trim off the ends to final size, and the base pieces fall right apart.
Wet sanding with 320 grit paper brings the sides and ends flush. Time to glue on the bases.
The containment pad to corral any errant oil. Just .040 styrene wrapped in quarter-round with a .060 x .060 curb.
The tank in its final position. A brass wire supply line with a Plastruct valve and N-scale brake wheel almost complete the model.
The two tanks side-by-side. Awww, father and son.
Completed model with the fill-pipe.