**cracks knuckles** I've been looking for ages on this.
I've seen a ton of milk crates. The rear rack dimensions are 409 mm × 477 mm or 16.1" x 18.8". A standard eurocrate sizing is 400x600 (15.75" x 23.6"), which would fit great - if centered, you'd have 2.5" hanging over each side. It's surprisingly difficult to "easily" pick up a single eurocrate in the USA; most of the places that sell them want you to buy a thousand of them, or they come in 3 packs, or you pay ridiculous shipping for just one.
Along the same lines, one of the better looking workbox toolboxes that fits the color profile and the size is the Milwaukee Packout XL Toolbox, 16.25"x22", so very close to the dimensions of the eurocrate. That has a QR interlocking system so technically you could bolt another one to the top of it. The guy that had one of these already had the Milwaukee stuff, so he was using what he had. Also with the fact that it has a mount, you will have a true waterproof solution, as opposed to a mostly waterproof solution. Let's face it though - if you're submarining the bike so far that your holes are letting in water, you probably have bigger problems. You can get those from Home Depot.
The Milwaukee Packout (rigid) Crate is 15.3 x 18.7" x 9", so the same width but slightly shorter and most importantly, in the right color. This would work well if you're a slightly taller person, would give you a little clearance behind your bum. Or if you're thinking extra fuel, the extra 1" lip on the base could provide some additional support to mount a Rotopax 2G (18.5 x 14 x 3 inches) to the side of the the crate while still supporting it. The biggest advantage to something like this is with the power of an electric drill, the ability to mod the ever living shit out of it and start attaching a billion things is yours. Being the packout system, you can hot swap it if you already own Milwaukee stuff. Of course, you lose the ability to have your stuff be waterproof because it's a crate, and you still need something like a cargo net to stop stuff from flying out while you're getting sick air from all those jumps you're going over. You can get the crates from Home Depot though.
If you're looking for something more standard and budget friendly, you can have the exact same thing without the quick release
from Amazon for around $20. It's even still within the color scheme of the bike. I don't know how accessible the underside of the rack is, but anything can be quick release through the use of wingnuts, if you get my drift.
If you're not picky and you like the idea of something collapsing, Costco or equivalent often sell Greenmade, Clevermade, etc. crates in a 12 gal size (14.25 x 20.8 x 2.75"/11.75"). These are usually pretty cheap, but I've been using one of these for years and it works great. The biggest problem I found is that you might need to have some lid retention, otherwise the lid can bump loose (it's friction fit). So for that reason, I'd try to find something with a latch.
As always, there's the ye olde apple crate, or something you can fabricate yourself, but with the price of cedar these days that might end up being a little more costly.
Something to be mindful of is the collective weight of all the gear. The rack weight "guidelines" are set at 20 kg or 44 lbs. This is mostly a handling thing, but Honda says what Honda says. So if you're looking at the basic crate (4 lbs), 2 gal of fuel (12 lb), the rotopax can itself (4.7 lb), the mount for the rotopax can (1.4 lb), then before you've put anything on the bike you're looking at 22.1 lb, leaving you with only 22 lb of gear left if you're going camping. Now, these are 'guidelines", but it's just something to think about when you're adding anything else on there. I know my full set of camping gear is 14 lbs but that's a setup that was designed for bikepacking, so it's all ultra light and ultra compact, thus also expensive.
This thread from ADVRider documents several people's ideas for what to put on the back.
Probably the only other thing to note is the paint scheme is SOFT. You're going to 1) want to protect the frame but 2) allow adequate drainage so you don't get water in prolonged contact with the frame. I don't have a suggestion here, but someone suggested a silicone kitchen mat. Others have suggested helicopter tape, and someone had some sort of foam protective tape. No matter what, they're all saying the same thing - spatially separate the box from the rack itself. (I personally am thinking that I will helicopter tape the upper surfaces, use standoffs, and mount everything to the standoffs, but that's also to mitigate rattling)