That looks like extremely solid construction. And heavy. I think having solid side walls limits what you can do, for example carry ladders.
At least for me, I don't need strength like that so building up from a heavy duty pallet was sufficient. My forks attachment and platform are each light enough to drag around by hand and I would rather patch or modify my lumber construction occasionally if needed, rather than work with an indestructible steel-frame box I can't lift by hand.
I think my YM186D (YM1510D) has similar lift capacity to your YM1500 so let's estimate total lift capacity at 550 lbs maximum for these. If that box weighs say half that much, it will limit what you can carry. In my photo above carrying water in garbage cans, (on the larger tractor) I started with five cans but couldn't lift them. That taught me to keep the back platform as light as possible.
I think the steel box in your photo is a specialty tool for a stonemason etc who needs indestructible durability. My own preference was to build light so I had weight capacity left for cargo. And I prefer no sidewalls.
I often use a couple of huge plastic bins to carry garden debris etc. I think its easier to tip these by hand to empty them, compared to shoveling out that box.
I suggest buy a made-up 3-point forks attachment, then get a big pallet and put a deck and front fence on the pallet. The forks will be more broadly useful. You can move other items on their own pallets using the forks, compared to lifting each object into that fixed size box to carry it.