Stopper for Moss Scuttle?

I have a large Moss scuttle by Sara Bonnyman, the first scuttle I got, which I like a lot but which does not keep the water as warm as do my Georgetown Pottery scuttles because it does not have a stopper.  The hole for filling the reservoir is not round, which makes finding or fashioning a stopper problematic.  Its dimension are approximately 42mm width by 19mm top depth tapering down to about 12mm.  I have thought about getting a very large rubber plug and cutting it to fit, but I'm not confident that it would fit tightly enough.  Does anyone here have any suggestions for a stopper for this scuttle?  TIA.

Well, I ended up trying a cork stopper, but it didn't seem like it would be very durable. So I got a black rubber stopper at a local hardware store, cut it in half, and then cut off another slice to size it to fit the scuttle reservoir opening. It isn't elegant, but it works quite well, keeping the heat in noticeably better than before.

That is a good solution, for the timing being, John.

John,

Try some self setting material that sets up flexible but firm.
A hard setting putty could also be used.

Shape it roughly and slightly oversize.

Then wrap iy in plastic wrap and force it gently into the opening.
Let it remain there for the instruction's set up time.
Then remove it and trim off the excess from the top and bottom.

You might even want to pinch the excess at the top to give you easy surfaces to grasp.

I have Just found this company on the internet. They seem to have a wide selection of useable products. Smooth-On.
https://www.smooth-on.com/ecatalog/mobile/index.html

Mickey

Thanks for the suggestion, Mickey. The black rubber stopper that I fashioned by cutting a round one to size has been working quite well. And I think that is the limit of my DIY skills.