That's what I have on my current boat. Previous boats kept it in the console of the truck, and a spare on the boat.

Have you considered purchasing (in your case 3) plastic couplers - from Lowes or wherever, glue them to a piece of plastic- that could be mounted on your trailer (only a thought) and then when you take the plugs out you screw them into the plugs- when you put the boat in there are there where you left them. I would seriously doubt if vibration would ever loosen them.

ONLY A THOUGHT

3pointstar

Thats what I was thinking, I use a crown royal bag now. I want something to hold the plugs and wrench that is discrete on the back of the trailer.
I have a shop that does custom plastic work, I might have them integrate a few caps and a hook

I use a 1/2" AN pipe plug because I like the look better. It takes an Allen wrench to install.

Drilled a hole le at the base of my winch going down, wrench fits in there and had an extra loop for a second safety chain (yeah who knows), it was the perfect size, tapped it out and my plug screws into there right below the wrench.

Cant forget plug if tow vehicles are switched, and wrench is right there in the winch when you go to unhook if you forget the plug you'll see it.

I always kept mine, in the seat back pocket of the truck with a wrench. And I keep a spare set in the trailer tool box within an old Swipes container, also with a wrench.

I suppose you could make something nice with either a 3D printer or wood, and actually mount the "receivers" into it that match the threads, as well as a slot for a wrench. You could make this to fit into a cupholder or other.

They also make magnetic drain plug wrenches which I kept one of those on the front winch stand.

I use a STAINLESS plug and had a SS wing nut welded to it, no wrench needed

I drilled a small hole and ran a wire through it and hang it in my keys when not installed

I have a garboard drain plate screwed to the underside of one of my bunks. When my drain plug comes out, it gets screwed into that. The wrench is bolted to a spring clip and hangs by an eye bolt right next to the drain plug. Everything is right there at the transom waiting for me when I get to the ramp.

THIS! Yes one of those little grey cylinders. AND another one in the glove box of the tow vehicle.