Lot's of good ways to build a box.
As for the freezer, I like those and they can be great and simple.
Some have plastic inner walls and some have steel. Either works but I would prefer plastic. I think for metal I would soak junk towels or other cloth in strong water and baking soda and let dry, line the bottom with that. If an acid leak the towels should netraulize it and help protect the bottom of the box.
I would make sure the inner metal walls are nicely painted or still has a good factory coating.
For wire inlets, drill a hole through side and glue in a piece of PVC pipe, this protects wires from cuts.
Insulation, already done. Add more inside or out as needed for your climate if you want. Already insulated to stay cooler in summer and warmer in winter. Get one a bit larger than needed, great place to store tools for the battery use, cleaners, brushes, wrenches if needed, etc... I would place a fake wall section inside to seperate storage area from batteries though.
Most the freezers I used outdoors for various things (or just let sit unused) have been fine for strength, solid lids ( I stand on them allot), they don't rust fast if bottom is painted well or is on supports off the ground. They do let rain leak in though so I put something over them, plastic sheet, piece of metal siding or such. Just anything to flow the water off the top so it does not run directly in the seals or between the walls.
A decent hole saw cuts through a freezer nicely so cutting holes for the Vent pipe or wires is easy.
Best of all, they are just a very large FREE box, and you are also stopping them from going into a land fill somewhere when you find them being tossed out on side of road.
Buying a good used one is cheap, maybe $20 for one that works good, but we don't need it to work. I think the good one I been using as a freezer for years now I paid $40 for at an auction.
Freezers may be great for many other things also. I have a medium size chest freezer I am drilling arm holes with PVC pipe inserts and airline holes in, installing a window in a side, outlet for shop vac on back, add a light and I have a mighty nice sand blasting cabinet larger and better than a $100 HF one, also nearly free 
I have a really large freezer sitting by the dry pond. Big enough to sleep in, might fit two tightly. I think 6' long! Wife was joking about filling it for a swimming pool when we hit 105F the other day here. Hmmm, her joke but my Idea, Hot tub?? I have to drag it to the house, check the strength of the walls when full of water. I think it will work ok but may need to run a support along the sides to prevent bowing out under the weight of the water. Take off the lid but keep it for a cover to keep leaves out.
Easy to paint a freezer anyway you want, so it does not have to be ugly sitting outside either.
