For 2018 California bike owners, I removed the charcoal canister, which freed up a ton of space under there:
Charcoal canister as it lives under the seat. Left hose goes to the air intake; right hose goes to the fuel tank. Not sure what the plugged nipple is for. There is also an open nipple on the backside of the canister.
Pop the clutch side under-seat plastic off and find the hose under it, disassemble it at the junction, and install a 6mm or 1/4" bolt into the engine side of the hose. Use the clamp to help secure the bolt.
Zip tie it up in there. Prop up the tank using the instructions in the manual. Follow the "other" hose (the right one coming off the canister) up to the bottom of the fuel tank (left nipple) and pull it off. Unhook it from the keeper under there.
Pull the canister up and out, and pull the hoses out. It may be easier to remove them from the canister and pull them towards the front of the bike. There is one reusable zip tie holding them to the clutch side subframe near the battery, unhook that as needed.
Using one of the removed hoses (or a new, similar diameter hose), attach to the nipple on the bottom of the fuel tank that you just revealed and route it down and to the throttle side bottom of the frame, near the footpeg, with the other vent / overflow hose that comes from the fuel tank. Forgot to take pics... sorry.
Reassemble. Trash your leftover parts, if you want. Keep them if you are OCD.
Done. Enjoy your underseat storage. You can use a razor knife to trim out the compartments, if you like; to make one big compartment.