Fat-fendered Ford battery tray
In most pre-50's cars and trucks, the battery was located under the floor boards or mounted in a tight location in the engine compartment. Batteries came in small 8X10X10 six-volt or long 4X16X10 sizes and access was limited, making removal or servicing difficult.
Rebuilding a car of this vintage meant opting for a 12-volt battery with more CCA (cold cranking amps) but how to fit it? Generally, the 'new' battery is installed in the trunk or mounted somewhere under the frame. Cables are then run up to the engine compartment, starter and fuse panel.
Here is an alternative placement that is not only close to the starter and fuse panel, but uses up some of the space we never think about: the area behind the fat fenders and front wheels of most 40's vehicles. A properly installed battery and tray in this location can function just like an under-hood or in-trunk battery.
 How to build it
Begin by making a small cardboard box, approimately the size of the new battery. The mock-up is easier to handle than a regular 40-pound acid-filled battery and making it an inch or so larger will cover additional clearances for cables and hold-downs.
Place the box against the frame area and try to imagine what kind of bracket will be required to hold it there. Duct tape can secure the box in place while flexible steel strapping (usually found on packing crates) can be shaped into 90 degree bends to support the battery.
The location should be easily accessible for servicing or removal yet protected from potential damage from stones and other debris. An accessory shield can be added to further protect the battery casing. Ideally, the frame of the vehicle should be close to this location as it is one of the strongest mounting points available.
"What about water and dampness?" you ask. Well, most modern batteries are sealed, maintenance-free units, 99 percent impervious to outside water sources and cable connections can be coated with dielectric grease to seal out corrosion. In addition, you can put your battery in a plastic marine battery case, which will provide both waterproofing and protection from physical damage.
A plastic marine battery case can be used in place of the cardboard box to find your location.
You can use cardboard to help design a battery tray and, by making a 90 degree bend in the cardboard, you can run it up along side of the frame. Use the frame to stabilize and mount the tray. Nuts, washers, brackets...etc can extend out from the frame, giving the tray a solid mounting surface.
To utilize the vertical side of the tray, a cutout can be made and bent 45 degree outward, adding a hole for a mounting bolt to create an offset for a mounting location to the frame. If laid out correctly, all three sides plus the bottom can be cut out of one piece of flat stock and bent to form the final battery tray. See pictures.
You can make trays out of 1/8 steel plate backed with 1/4 angle iron and flat stock, as well as stainless steel and aluminum. What you use will depend on what you have around and the equipment that you have to bend it and fasten it together. The box that you see here was made using a six inch vise and angle iron. Stainless steel makes a nice, bright, maintenance-free battery tray. Aluminum is easy to bend and drill, but requires a TIG or MIG equipped with pure argon and either a teflon liner or a spoolgun to weld on. Not all migs can acept a spoolgun so check your owners manual for details on welding aluminum. Combinations of any of these metals can readily be used and will make a nice strong tray.