Twenty minutes ago you were trying to get something started. Now it is going, properly going, and the rice needs draining.
So you are the one who has to end it. Not because anything went wrong, but because dinner exists.
First, what this is not
Worth separating out, because the three situations look nothing alike from the inside.
An activity that stops working is one thing. A child who has wandered off to something else is another. This is the third one: it is still going, and the reason to stop comes from outside the play entirely. It is the bath, or the door, or the fact that somebody has to eat.
In that case a countdown covers one thing only. Five more minutes says when. It says nothing about what happens to the half-built thing on the table.
Say where the thing is going
The move is small. Alongside whatever you say about time, say what happens to the object.
It stays where it is. It goes on the shelf, still assembled. We take a picture of it first. The lid goes on the box and the box comes out again tomorrow. You do this one last bit and then we are done.
That is it. You are not making the ending nicer, and nothing here is a promise about the ending itself. You are stating out loud what happens next to the object, in a form you can follow through on. If you say it stays on the table, it needs to still be on the table later. That is the part that asks something of you afterwards.
One thing this is not for: if they have moved on to something else and you are the only one still at the table, that is a different situation, and it is covered separately in stopping early rather than here.
The four versions
Leave it out. An option when the space allows it. This stays exactly here until after dinner. Nobody moves it. Available for anything flat, dry and clear of the walkways.
Move it whole. For when the table is needed but the thing itself can survive a journey. A tray, a spare shelf, the top of a chest of drawers. It’s going up here, still built, and it comes down in the morning. Name the actual place rather than saying somewhere safe.
Photograph it. For the cases where nothing can stay out. I’m taking a picture before this goes back in the box. What is left afterwards is a picture rather than nothing.
One last named action. Finish that wall, then we stop. Do the last one and that is us. Name one specific action rather than one more turn, and be ready to treat it as the end once it is done.
Some things have to be cleared now
Being honest about this, because leaving things out is not an option in every house.
Food goes away. Anything wet on the floor goes now. Anything in a doorway or on the stairs comes up before somebody puts a foot through it. If it’s small, breakable and there is a younger sibling in the house, it does not get left on a low table overnight. And if the setup takes up a surface that somebody else needs to use, that is not really yours to leave. If it is spread across a rug that somebody wants to hoover in the morning, the same applies.
None of that changes the approach, it just narrows which version you use. When nothing can stay out, the photograph is the one that still applies. Take it, say that is what you are doing, then clear up.
What you actually get
Not a smooth ending, and not a prediction about how any given ending goes.
What it is is a decision that belongs to you. Ending play that is going well does not require pretending it had finished. You can say plainly that it was working, that it has to stop anyway, and that what is on the table is going on the shelf until tomorrow.
Then put it on the shelf.