You don't *Need* diamond plate, and much of it would not be visible even if you used it. I did a quick Google search and a ready-made 18x18x90" steel box was $1000 at Tractor Supply, so it sounds to me like that price isn't unreasonable. You're getting one twice as tall and quite a bit deeper, and rust proof aluminum.
I don't know if you'll need mounting brackets or not, or if the ready made box came with 'em or not, but consider how you'll mount it. Hanging it from the floor is *NOT* sufficient! The 'floor' absolutely needs support from the ends/corners at the very least, and at 88" long it *WILL* desperately need some floor support in the middle as well, with any amount of weight in it.
We run these type of boxes on our flatbed trailers, I'm not sure if they are 48 or 60" long, but it's not unheard of for each one to have 300 pounds+ of spare straps, chains, binders, corner protectors, dunnage (wooden blocks), leverage bar (for tightening/loosening the straps), tarps, bungees, traffic cones, nuts, bolts, washers, screws, odds, ends, knicks, knacks, paddy-whacks and countless other random bits and stuff accumulated over the years. Without the strong support, the aluminum would flex, crack and break within a few months.