The band van is a repurposed airport shuttle, so the back door opens into a space under a single bed - ie. 1 x 2 meters, and about 90cm high. I'm calling that space a "box" but it's basically a space of fixed size.
Thanks for the link - that's awesome. This one is housing a full kit, right?
https://www.roadcases.com/2-sided-combo-drum-ata-case-w-adjustable-shelves-202046/
So outside dimensions without casters is 25 cu ft which matches my calc and has a fair bit of space for padding. (Again though, I'm more interested in total volume even if it's dispersed amid the band's other gear, rather than a neat box-shape.)
It’s probably not housing a full kit, at least not at common dimensions. It could likely only accommodate up to an 18” bass drum, and that might not work with some drums. If that’s a tight 20”, cymbals might be an issue as well. Probably not very viable for most hardware either.
I was mostly offering a visual aid. Your prior reply had me again imagining the wrong scenario, this time in a full size tour bus with underbelly storage. There are other road case manufacturers if you want to tickle your eyeballs with other examples of how to stack gear.
But now, finally, I see that you’re in a large sprinter-esque van, with a converted sleeping loft in the cargo area. (I really hope you have at least two feet of height in the coffin). I wouldn’t recommend large road cases for that scenario; better to be closer to ground or with a ramp. That single item weight and volume is going to get old quick.
For your purposes, soft cases should be fine. I’m catching hints that your budget is tight (“wrapped in blankets”), but even something like appropriately sized cardboard boxes with some kind of padding *should* be fine, and will be a lot easier to slide in and out of the bed of your cargo area. This is also assuming you take measures to keep your cargo from shifting while in transit, and how much your drummer cares about their shells. Plywood tubes reinforced with rigid hoops are rather resilient, though.
If your dimensional and monetary budget allow, add a hand truck of some kind.
Your rough estimates of volume look good, and your rounding up should provide you plenty margin for error.
I’d recommend you spend some time with graphing paper or a digital grid, hell, you could 3D model if you’re that savvy, and slide some shapes around after getting some specific information from your (bandmates?) on what they’re planning on bringing. It’s also good to take a day to make sure the arrangement works in practice. Sucks to have to cut things for space when it’s crunch time, better to know in advance.
Regarding case dimensions, hard cases are generally going to add 4 inches in both directions. As a quick example, an SKB Roto-X 14” and 22” case for snare and bass drum come in just under 18” for the former, and 27” for the latter. Soft cases are somewhat variable depending on the material and quality of padding, but still allow for some compression.
My condolences regarding you drawing the logistics straw.