You've just got to keep looking. It took me 3 months to find mine, I got it off ebay. Measure your truck carefully and know what you are looking for. A toolbox to sut my truck was going to cost me about $4k. I got mine used for $200. It's all aluminum, goes side to side, fits my truck perfectly and has shelves and pneumatic lifters on the doors, came with all keys. In mine I carry 6 saws, climbing kit, all my ropes, all my rigging gear, 3 large lowering ropes, helmets, earmuffs and a crate full of misc crap. It's completely waterproof, very light and works perfectly. I've worked in my younger years as an aluminum welder fabricating plate aluminum boats and truck bodies but I couldn't make something this good. Mine was also powedercoated white and in near new condition.
You're kidding yourself if you think you can make your own toolboxes and have them be light, functional, waterproof and not look as if you made them yourself. I'd built a lot of trailers and truck tipper trailers working for companies and built a few small trailers for myself. I always wanted a tool trailer for all my gear but baulked at the high price ($5k+ for a good one) compared with a similar size box trailer at $600 new. Built my own over a period of weeks which stretched out into months. I had access at the time to a press brake, sheet metal guilotine and all the usual hand tools and welding gear. I had to pay for all my own steel and axles, mine came out weighing a lot, and the shelves, sliders and doors just weren't quite perfect of perfectly waterproof. I spent about $3k on materials to end up with a trailer I couldnt sell for $1k.
Buy good quality used. If things dont work out you can always sell it on.
Shaun