Here is a video of the thing in action.

This design is licensed as non-commercial. This means you cannot sell this. Please respect that.

I had my eye on the brush rinser from Green Stuff World, but I didn't like that it wasn't possible to close the bottle. Furthermore, it sold out before I could get one.
As such, I set out to design my own.
The core design idea was to have one that could be glued onto a soda bottle lid and screwed on there for easy water storage and emptying.
After a few iterations, I arrived at one that works, has a texture to make rinsing the brush easier, and flushes by a lever being pressed down.

Hint: Put some small in the bottom if you use a bottle that doesn't have a very sturdy base. Until it's full of water, this is very top heavy and tips over easily. I am not saying I tipped it over on my keyboard, I am just saying that for unrelated reasons, I sat yesterday and took off keycaps and switches, drying water out of my keyboard.... You could use steel too, but that is gonna rust over time. Sand is gonna work too, but do I need to point out why emptying it when it has sand in it would suck? I just went outside and took some small rocks from our driveway.

External hardware required:

• Glue
• An M3x25 bolt(or longer)
• A soda bottle(or whatever else you would want to glue this on, your choice)
• A rubber washer that's Ø19 and about 1.85mm thick. *
• A spring. I used an Ø5.7x19 spring. I wouldn't use any shorter than that.
• Optional: Vaseline to add to the surfaces you are sealing. I don't know if this helps, but it doesn't hurt, and in my head it does help.

Assembly instructions:

• Start by printing the model. None of it actually needs support if your printer is properly calibrated. Make sure everything that has a flat face has a flat face touching the build plate. This is especially important on "V4_RinseBowl.stl" as that needs to make a tight seal with the rubber washer. I originally printed that in one piece with the "V4_RinseBowl_Thread.stl" part, but the overhangs from the support caused it to leak.
• Cut a hole in the lid of the soda bottle or whatever you decided to put this on. Then glue "V4_RinseBowlPlug.stl" onto said lid. Make sure it isn't totally sealed, you need a bit of air to be able to get in or it won't properly flush the water. However, don't just leave it gaping open or the water may run everywhere. If you prefer, you can glue a perfect seal and then just drill a little hole in "V4_RinseBowlPlug.stl" to let in the air.
• "Glue V4_RinseBowl_Thread.stl" to the underside of "V4_RinseBowl.stl". This is where the rubber washer will be seated.
• OPTIONAL: Apply some vaseline to where the rubber washer will be seated. Apply it on the waster itself as well afterwards.
• Put in the washer
• Put the M3 bolt through the "V4_PlugSpringholder.stl", put the spring around the bolt and screw it into the "V4_RinseBowlPlug.stl". This model comes with an M3 thread pre-designed, but it is usually helpful to have "dry screwed" into it first to make up for any sagging, shrinking or other thread mangling of the 3D print process. Tightening the bolt as needed, do some trial and error.
• Screw the rinse bowl on the now prepared bottom part, getting the plug lever passing through the bowl. It should screw on with no issue. Don't over-tighten though as this is a very short thread and you printed it in plastic.
• Pour in some water, let it sit and check for leaks. If any leaks are there, the spring may not be tight enough. If the issue isn't the tightness of the spring, look for anything that may cause the seal between the bottom of the bowl and the rubber washer to not be properly sealed. This is designed so the plug pushes hard against the rubber washer, causing a water seal. Once you have made sure there are no obvious leaks, leave it to sit for a few minutes to test if the water runs through on its own slowly. If it does, there is a small leak still. Once all leaks are eliminated, press down on the lever to test the flushing mechanism. The water should run out on its own like in the video. If it does not, try pushing the bottle(if soft) and letting go. If a vacuum then proceeds to pull the water into the bottle, this means you didn't leave a proper air hole when gluing the bottom thread onto the bottle cap. This can be either fixed by breaking that seal and allowing in air or simply accepting that this is now a manual vacuum flush where you have to push on the bottle to make it work!

*I am sure you can substitute the rubber washer with some other kind of rubber, but I made this to fit a rubber washer and as such, I cannot speak to how water tight it will be if you don't use that. If I had to give a suggestion(THAT I CANNOT PROMISE WILL WORK), it would be liquid silicone or cutting out a bit of something like an old RC wheel or whatever. Maybe an old silicone sock for a 3D printer. However, if you have the option, I strongly suggest just picking up a rubber washer.