Measure the length, height and width of the oil tank to give you some idea of the size of your needed wall. Jot the numbers down and order or cut the lumber to those specs, adding 1 foot to the overall length and width to provide a safety buffer; you don't want anything to rest snug against the tank for fear of accidentally causing any scratches or scrapes that might damage the tank.
Lay a 2-by-4 on its thin edge, flat on the floor, flush to the wall and parallel with the tank's width. Lay another 2-by-4 parallel to the tank's length and have this board intersect with the board on the width to create a 90-degree angled corner. Screw the two boards together at this corner using rust-resistant galvanized wood screws. Lay another board along the opposite side of the tank's width and intersect it with the length board; screw the corner together to complete the wall's base frame.
Slide shims underneath each corner of the frame (which will wobble slightly due to the slight gradient of the floor, which allows for drainage.) Measure up from the floor, past the shim and to the top of the corner. Take this measurement and transfer it to the width side of two of the plywood sheets. Saw each sheet with the power saw, which will create a stable wall that conforms to the gradient.
Place each sheet with its curved edge against the floor, flush to the width boards on the frame. Screw them into place. Place another plywood sheet long ways against the long 2-by-4 and screw it into the frame with the screwdriver. Screw the corners of all pieces together to make the walls.
Measure the distance along the top of the tank to the fuel gauge's location. Mark the measurement on the remaining plywood sheet and saw out a circle wide enough to accommodate the gauge. Lay the sheet over the tops of the walls, carefully thread the gauge through the hole and screw it down into place.